This situation doesn't apply to me, was just wondering about any possible reasons behind it happening.
Most Helpful Guy
This usually occur when the guy goes into the friendship hoping for it to one day escalate into a relationship. However when the girl tells him that she doesn't want a relationship with HIM, then the guy's entire reason for befriending her in the first place becomes moot and he see no reason to continue.
Another reason he wouldn't stay friends is because it's much harder to get over someone emotionally when you continue to be friends with them. Remaining friends with someone who you like but they don't like you back is torture, especially when that someone eventually finds a BF/GF and you're forced to stand on the sideline and witness it all go down.