Why do guys get distant all of a sudden?

I started seeing this guy who has been chasing me for a long time but I never had any interest in him. Then I started spending more time with him and I really began to fall for him. He was so nice and caring and seemed like he was really into me! He knew that I was into him to because I couldn't help but show it.

Then, all of a sudden, within one day, he went from being interested, to just suddenly being very distant and 'cool' with me. This all happened over a matter of days so I'm very confused about what suddenly changed his mind.

I know he's pretty upset at the moment about his brother moving away as they were very close. But is this the reason he is so distant with me all of a sudden? Or has he suddenly just had a change of heart?
Why do guys get distant all of a sudden?
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