Why do guys ditch/bail suddenly with NO explanation?

I'm in college. This is how the past THREE "relationships" I've had have gone:

first day: meet for first time, text everyday, hangout, guys pay for meals, etc.
2 weeks in: start having sex on regular basis.
4 weeks in: still having sex but guy asks me to be exclusive WITHOUT the label of boyfriend/girlfriend, hanging out outside of bedroom. Guy tells me how much he likes me and wants to keep seeing me.
6 weeks in: hanging out regularly, remain "exclusive" as he asked for, but no real "couple" stuff (PDA, meeting friends/family)
8 weeks in: guy suddenly loses feelings, ditches and bails randomly with no real explanation, never speaks to me again.

Is this some new scheme for being friends with benefits or are these guys actually genuine when they say they have feelings? This crap has made me paranoid about every guy I meet and reluctant to talk to a guy more than 2 weeks.

Why do they bail all of a sudden and lose interest at a random point in time especially when THEY ask for exclusivity?
Why do guys ditch/bail suddenly with NO explanation?
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