The Colombian civil wars have happened for a variety of reasons. Let me just focus on the most recent two, as I don't know enough about the earlier ones to make a meaningful comment about it.
La Violencia (1948-1958) and the Colombian conflict (1964 - present) both broke out over a similar reason: unequal distribution of land. Both conflicts also developed along comparable lines. Peasants uprisings to violently seize land from landowners, followed by the formation of paramilitary groups by the wealthy to fight back, leading to increased professionalisation of peasant forces and the intensification of conflict.
Colombia has historically had one of the most unqual distributions of land, with a small elite group of landowners owning nearly all of the land in the country. A large number of non-land owning peasant farmers were unhappy over this and rose up in arms to seize control. Due to poor land titling and property rights, these peasants could rely on little support from the state in their grievance, and thus were driven to taking up arms, as larger companies and wealthy land owners were keen on land grabbing, with the peasant farmers having no legal leg to stand on due to a lack of deeds or property rights.
A additional reason for these conflicts to spiral into outright civil wars are the lack of state penetration into the fringe areas of the country and the weak military power of the Colombian state. Weak radial institutions of the Colombian state in the most remote areas, institutions such as tax-collecting institutions, transportation infrastructure and military and police presence, led to the rooting of these left-wing groups as legitimate actors in the country rather than small insurgent groups. Left-wing groups were able to seize large areas of land, over which they exerted control in a state-like fashion. These groups levied taxes, built hospitals and schools, and even judiciated in some cases. This led to a deterioration of perceived legitimacy of the state in these areas, which then subsequently increased support for these left-wing groups from the peasant farmers living under their control. The Colombian military was not strong enough to fight these groups and thus their power and legitimacy grew, allowing the conflicts to grow from small insurgencies to outright civil wars. Through Charles Tilly's theories on state formation, we can argue that groups such as the FARC were in the process of forming a state, or at least managed to establish a situation of multiple sovereignty. This multiple sovereignty and the Colombian state's inability to project power into these hinterlands is made greater by the lacking physical infrastructure leading into these parts of the country.
The Colombian state army's inability to deal with guerrilla groups in the hinterlands led them to de facto rely on paramilitary groups, giving away their monopoly on legitimate violence in large parts of the country, further undermining state legitimacy.
State legitimacy was also undermined by the so called 'false positives', were the Colombian army murdered civilians and dressed them up in guerrilla outfit in order to claim rewards for killing guerrilleros and to inflate the numbers.
All of these factors undermining the state's legitimacy in the eyes of peasant farmers, many of whom lived under the control of non-state actors in both La Violencia and the more recent Colombian conflict, led to an increase in perceived legitimacy of these non-state armed groups, which exacerbated the civil war and the armed struggle of these groups was seen as just or righteous. It also increased the "social capital" of the armed groups, where they would have the support of civil servants, politicians or other "friends in high places" due to their perceived legitimacy. (This in contrast to their "violent capital"). Some territories were even deemed "lost" by the Colombain government and the non-state groups were left to govern as they pleased.
These were grievance factors which exacerbated civil wars in Colombia, but there were also factors of greed influencing the violence. Non-state armed groups taking up the fight against the government need to fund their activities. This happens by exploitation of resources such as gold, cocaine production and drug smuggling, and other criminal activities such as kidnapping for ransom. These sort of activities beget violence, as numerous non-state actors vye over control over gold mines and coca plantations, as they are lucrative sources of revenue. Contrary to what many might think, gold has been the number one resource of non-state armed groups (including FARC) since ~2004, rather than coca.
Thus, the two most recent civil conflicts in Colombia started over similar reasons, and worsened over similar reasons. Unequal land distribution led to peasant unrest, weak land titling and property rights led to little state support, lack of state power in the hinterlands led to an increase in legitimacy for these non-state groups, which then started acting as quasi-state entities. Added to these grievance factors were the greed factors of control over lucrative resources such as coca and gold.
Sources:
Must read:
Bushnell, D. (1993). The Making of Modern Colombia: A Nation in Spite of Itself. (For this mostly Pages 201 - 286, and especially pages 205; 254-256)
Further:
Forero, L.F. & Urrea, D. (2016). Territorial peace and land grabbing in Colombia. The Transnational Institute. Retrieved from: https://www.tni.org/en/article/territorial-peaceand-land-grabbing-in-colombia
Grajales, J. (2011). The rifle and the title: paramilitary violence, land grab and land control in Colombia. The Journal of Peasant Studies.
Ibarra Sanchez, D. (2017). Illegal gold mining fuels violence in Colombia. Al Jazeera English.
Idrobo, N. Et al. (2014) Illegal Gold Mining and Violence in Colombia. Peace Economics, Peace Science and Public Policy
McDougall, A. (2007). State Power and Its Implications for Civil War Colombia. Studies in Conflict & Terrorism.
Saab & Taylor (2009). Criminality and Armed Groups: A Comparative Study of FARC and Paramilitary Groups in Colombia. Studies in Conflict and Terrorism.
Also check out:
Jennifer S. Holmes & Sheila Amin Gutiérrez de Piñeres (2014) Violence and the state: Lessons from Colombia, Small Wars & Insurgencies, 25:2, 372-403, DOI: 10.1080/09592318.2013.85793
Lujala, P., Nils Petter Gleditch and Elisabeth Gilmore. “A Diamond Curse? Civil War and A Lootable Resource.” The Journal of Conflict Resolution 49, no 4. (2005). Stable url: http://www.jstor.org/stable/30045130
Nazih Richani (2007) Caudillos and the crisis of the Colombian state: fragmented sovereignty, the war system and the privatisation of counterinsurgency in Colombia, Third World Quarterly, 28:2, 403-417, DOI: 10.1080/01436590601153937
Philippe Le Billon (2010) The Geopolitical economy of ‘resource wars’, Geopolitics, 9:1, 1-28, DOI: 10.1080/14650040412331307812
Reardon, S. Colombia After the Violence. Nature Magazine. May 9, 2018. Retrieved Via: https://www.scientificamerican.com/article/colombia-after-the-violence/