Why does my hair "clump" toghether?

Hello!

I (male) have fairly long hair, imagine my hair like one of the Beatles boys. So on the morning I shower, comb my hair and it looks very good. After a few hours outside my hair starts to kind of "clump" toghether and it looks like a mess. What's the reason behind that? I want to keep my hairs seperated and not toghether.
And also another question while i'm still at it: How do I make my hair light? Right now it tends to kind of stay flat, but I want a more "fluff" to my hair so I don't look like Justin Bieber did before if you know what I mean.

Thanks for your help!