Why do I feel like logging into my ex's FB account?

This is long please bare with me.

My ex and I dated for 2 years. He was very serious about the relationship and did talk about the future a lot. I am taking a gap year and then leaving for university and he was always so sure about LDR but then he changed his mind and we broke up mutually 6 weeks ago. He was very upset about it and broke down the day before we broke up saying he can't lose me. But we decided to stay friends.

2 weeks after the breakup he came to meet me while I was with my friends. Another 2 weeks later I called him up to ask him why he changed his mind suddenly. He said he had been thinking about it and spoke to his family too. He took the decision after thinking for two days. He answered my qs casually n told me he has moved on quite a bit n goes out a lot so it was easier for him and I am taking a gap year so thats why I am still stuck.

I asked about whether he was open to a relationship in the future n he suddenly changed his mind, said sorry but no, coz its "weird". He said its not like we aren't going to like/be with other people. He even mentioned he does find some girl attractive (after I kept forcing him to tell me). "I know it seems fast but this is who I am, why can't we just be friends, etc etc". I can't believe he is already onto the next girl in 4 weeks. He is so different all of a sudden given why we broke up. Its very unlike him.

I am unable to accept this because it hurts so much and I wish he is just as hurt as I am and misses but he doesn't care about me probably so I am tempted to check his FB but I won't because its wrong. But why do I feel like this? What do I do?
Why do I feel like logging into my ex's FB account?
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