The answer is little bit complex, Athens has today the image of the open and free democracy, centre of culture and trade of the Greek people. Well Athens loved democracy in Athens, but not in the Delian league. Athens with its league was not like the peloponnesian league an alliances of multiple poleis. It was an empire, starting as league against threats like the Persian the Delian league under the control of Athens became more and more an Athenian Empire. Athens formed the largest fleet and army of the league and demanded that the other members provide ships or money, as most could not provide ships money was send which payed for the Athenians fleet. So, one understand that Athens was not interested that members would leave the alliance or not pay thier contribution. It used often the fleet that was payed by the league to suppress or punish poleis that wanted to leave or were not happy by Athens hegemon. When poleis were punish they were often forcefully democracies, and Athens made sure that the demos followed a pro-Athenian policy (even if the demos was only pro-athenian because they feared the oligarch and nobles would punish them if they returnwd to power). Another method was to establish a rival Arhenian colony on the land of the Polis making sure they stay in line. Inshort it was an empire that cared only for it own power and the suppling Athens. Every thing that question or threatend it that power or interest is threat and had to be eliminated.

Another element is that the Greek societies were extreem militaristic. Sparta is the most extreem example as it made every male citizen a soldier. However Athens was itself militaristic and so expansionist. Warfare was essential social and economic practices for the Athenian citizens. More important to become a citizen and have voting right in every greek polis you had to serve and fight for the polis. However not every free native male polis inhabitant could fight, he had to provide his armour and weapons. The hoplite, the main battle unit, was more expansive to be equiped, so only upper class and upper middle class could fight and so have voting rights. Athens did not become a democracy because it gave all its male citizens voting rights, it became a democracy as the lower classes got accepted as rowers in the growing Athenian navy. So participating in war and getting voting rights.

This brings us to another element, democracy itself. Like said earlier, for the greeks warfare was also an economic affair, as raiding was normal and happend regularily and the plundered spoils of war would bedivided between all participating citizens, depending on ones achievements and rank during the war. Now imagine that that a majority of the voting population has a low income and were day laborours. Now the spoils of war was a major injuction of wealth and the legitmation of ones voting right. So, as poor labour would you vote for wealth and voting rights or for peace? Well most of rowers would vote for war, we can see how this democratic dymanic caused Athens to go more often to war than other poleis. Although greek quite frequentily went to war.

The greeks had a strong proto-patriotic relation with thier polis, the honour of the polis and even wrongs comitted in one war could start a war. The Argives excuted Thebans rengates who attack during a holy month, to revenge those the Thebans attack and a war started where Sparta was pulled into. When the Argos was taken by them the Argives justified themslevs as the they justly excuted the Thebans. Even twists or feuds between individuals from different poleis could escalate inro war.

This leads to the relations between Sparta and Athens, or the Peloponnesian and Delian league. Both poleis had rivalries with each other and other poleis that rivaled Sparta or Athens or rivalt a polis that was allied with one of the two aligned one with other alliance. Furthermore we are speaking of two powerful alliances close to each other and bordering spheres of infleunces. In such a situation escalation is question of time. One can compare the situation with a mix of the relations between USA and USSR in the cold war and France and Germany before WW1. Add to that Athenian empire building and desire to establish a Hegemon over Greeks poleis and militaristic and war mongering lower class, and you have your answer.