In Eric Christiansen's Northern Crusade, he makes the point that membership in the Tuetonic Order went along with many of the hardships of the monk's life (strict social and behavioral regulations) along with those of the knightly life (death, etc), and that while the monastic vows were not perfectly followed they were taken seriously. He also noted that at least in the first century they drew from a very wide number of social bases, from lords to merchants, which made me wonder what the biographical details of the individuals were who voluntarily chose such a difficult life.
Bernard of Clairvaux extolled the military orders because they provided the chance to follow typical knightly pursuits for a godly cause, and a redemption for those who had shed blood for worldly causes. Was this generally persuasive (to pt a slightly romantic gloss on it, were many of the recruits former soldiers who felt the needed to atone for participation in secular wars)? Did the recruits tend to be veterans or young men looking fr a proving ground? Was there something like the stereotype of unpromising children being sent to the monastery?
I only have one source, and I hope that'll do; Thomas Asbridge's The Greatest Knight: The Remarkable Life of William Marshal, alongside Peter Coss's The Knight in Medieval England 1000 AD-1400 AD.
Put bluntly the answer is faith. Put less bluntly - it's hugely complicated. Because 'faith' encompasses a great many motivations. Some of the scions of various Frankish, Norman, and German families joined because they had little chance of inheritance back home and a monastic knightly order offered security, activity, and religious fulfilment - this last likely being the reason William Marshal was made a templar on his deathbed. Others joined because of a genuine sense of religious fervour, in the same way military volunteers have fought in 20th century conflicts. They believed ardently what they were doing was right, and that by fighting for the Holy Land, and protecting pilgrims, they were offered the opportunity to truly serve God. In a world completely immersed in religious teachings, where the church's power was growing quickly, it makes a great deal of sense for it to attract all manner of recruits. Alongside this, we must remind ourselves that the High Middle Ages, while far from a backward and regressive time, was still a harsh period to live in when directly compared to the modern era. The life of a templar recruit could be relatively simple when compared to the backbreaking work of a farmhand or the constant financial insecurity of a travelling knight searching for a lord - though do not overstep in saying that either of these professions were exceptionally hazardous ones in this period.
I can find no evidence of the Knights' Hospitaller, the Teutonic Knights, or the Knights' Templar being a 'last resort' for unwanted children. They were, as Bernard of Clairvaux, considered prestigious and wealthy orders right into the 13th century. They served with skill in the defence of the Kingdom of Jerusalem, and developed systems reminiscent of early banking.
In summary, I will say that the most common recruits for the knightly orders TENDED to be the landless sons of nobility; trained to fight, they had many options, one of which was to serve God in the Holy Land. However, for the servants and attendants of the orders,it could be anyone, ranging from local hands to pilgrims who visited the Holy Land and elected to say. The call of religion was no small thing, though I have no idea if any joined to atone for secular wars (I personally think it very likely, but without evidence, I will say no more). It was a way into Heaven, after a lifetime of sin. It's easy to see why many joined.