-He can't explain because he's type of guys who is not good with explaining and using words and he can't express himself well and how he feels.
-He feels complicated with his life, he likes you but in the same time he might think that maybe it's just a temporary phase and it'll pass, he might stuck with if he really likes you or not and been caught in between of his purposes and you and everything in his life cause he couldn't find himself yet and he's not sure what he wants to do, his life is yet unrecognized in his eyes which makes things hard to decide and even harder to explain.
-He couldn't explain because he didn't wanted to tell the truth, whether likes you or not, he wanted to hide the feelings and by your question he was busted and he only found the words "it's hard to explain" to just makes you move on from it.
This is the three aspect i could think of right now. It could be any of them. It's up to you to understand it. You can know more by talking to him more.