How do some people date so many people?

I'm in my early 20s and I've never been in a relationship, mainly because once I'm into someone I'm in it for the long haul. I also have a hard time letting things go in general. When I was in middle school I had a crush on this guy for 4 years. He was much older than me and I was too shy to do anything about it. He probably still doesn't know that I exist. Then I entered the college phase. I fell in love with my neighbor and he didn't reciprocate. It's been almost 2 years since he rejected me and I still haven't moved on from it though there are several guys that are into me.
I know there's a lot of 20something who's already been in 3 or more relationships.
If you're one of them, how do you do it? How can you move on so fast? (I'm not asking this in a sarcastic way, I'm genuinely curious because I want to be in love too)
How do some people date so many people?
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