You mean because "he's a guy and that's what we do" isn't answer enough?
Please give us an example of his behavior and we can answer you question more accurately.
This is a different person and I think it is most likely because he likes you but he knows you don't like him. He wants to ask you out but he knows there is no use. So he starts acting depressed and weird and unhappy and grumpy and hangs out with you less often.
Girls just find guys weird because they act silly to impressed girls and they're girls not guys.