i need help understanding why guys cheat and it doesn't matter to them how much the other person cares for them or what they've been through together

no reason at all

Explanation: guys only cheat and especially towards a younger age when they haven’t fully developed in maturity, men at all ages will cheat. And after a period of time they finally develop proper emotions but don’t have the same perspective of women. Their brains don’t process the issue the same way as we do.

If you are personally asking because this is happening then I would suggest waiting, focus on studies and when you meet someone. Talk to them for a good solid month before making anything official. Time is the best development of meeting someone because you will learn who they are as a human being rather than an object to sleep with.