tl;dr: answer at the very bottom
Like Honestiago5 said:
or they are demoralized so much they no longer have the tenacity to keep trying.
Dating for me has gone like this:
[Lots of rejection throughout all this]
Random shit in junior high ("dating" - holding hands, some kissing, nothing serious)
One girl says she has cancer to see what my reaction will be. She saw it on TV, and figured it would be a good test. I failed that test.
Different girl I'm into comes up to me out of the blue one day and asks me out (roughly 10th grade - age 16-17). Ohshityes. Turns out she was paid $100 to do it. :(
Junior prom date just wanted to get into prom and hang out with her friends in a corner while drinking punch.
Senior prom date was normal but then I went away for a semester, ???? happened, and when I came back she was a stripper. And not even a good one (from what I heard) and she tried to blame it on me.
Met a chemical engineer in calculus because we got our meningitis shots at the same time. Neat girl, but she texted me while I was in another class one day. She ended up talking herself into our breakup before class was over. Afterwards I texted "I was in class..." and she was all like "omg jk are we still going to the roller derby on Friday" to which I said "I don't go out with people who break up with me" (sadly, this is probably one of the high points of dating for me)
Dated a computer science girl. Fell madly in love. She broke up with me on finals week through a text. Refused to acknowledge my existence after that point.
Dated a girl for ~2.5 years - longest relationship, but it just wasn't going anywhere and the most we could say about each other at the end is "it isn't bad".
It's just so much shit. All the time. And it seems women make it a hobby to find new ways of rejecting guys. I get why - they get asked out a lot, so they have to reject a lot, but a lot of women treat it like a game. There's a lot of mocking phrases used like 'oh, real original' (which is ironic because it's not a very original rejection) 'not even in your dreams', etc. Those are appropriate for some places like a bar - the guy just saw you, there's not much invested in the situation on either side - but it's brutal to hear that from girls you see regularly.
How the hell do you deal?
All of the above. Tons of bullshit. Tons of practice. Eventually your give-a-damn breaks.