Dean Winchester pushes people away like it’s his goddamn job (which it kind of is, if you think about it). He never admits to needing anything other than family, and even then it’s difficult for him to admit it. It’s rare that we see a scene where he’s open about his feelings, and when he is open about it, he’s not exactly pleased to be saying it. He’s guarded even when he’s not.
When Dean Winchester says ‘I need you’, he’s saying something more than that. He’s saying that he can’t live without you. If you think about it, ‘Need’ is raw and desperate. You can love someone without needing them in your life, but you can’t need someone the way Dean needs Cas (or Sam) without loving them. I think that saying ‘I need you’ (not for the first time, mind you) to Cas is Dean’s way of admitting not only to himself, but to Cas as well, that he truly cares about him.
Maybe Dean doesn’t believe in ‘love’ as we know it. His entire life has been about loss and the absence of people who he cares about. He’s never really known what love is and he can’t tell people that he loves them because it’s foreign to him, and it sounds wrong coming out of his mouth.
'I need you' is Dean's way of saying 'I love you'.