For a long time I was hung up on what "I love you" actually meant. I found it incredibly frustrating. If a girlfriend told me she loved me (back in my mono days) I would ask her to define love, because it didn't seem so much an emotion as a request for more commitment. I felt that if I said those words I was saying "I don't want to be with anyone else besides you, and I don't see that ever changing."
I researched how our brains act when we are developing a new relationship, about infatuation, friendship, serotonin, oxytocin... trying to identify what being in love
actually was. I figured what I felt was love, and what the girls felt was love, was actually just infatuation. At that point I felt even more frustrated because I didn't think they could possibly be genuine about what they felt, and yet I didn't want to drain them of that wonderful energy and feeling they had by telling them.
Lately however, I've had a different experience: I took my last shot at a serious monogamous relationship two years ago. I probably went on about 40 first dates with women I'd met online. Finally I met a wonderful girl, and we dated for almost a year before I got bored and ended it. We started having sex again a couple months later, and have been on and off for the last year. A couple weeks ago we started saying "I love you" again, even though we aren't thinking of getting back together in the traditional way. The thing I notice now though, is that there is no guilt, no pressure, nothing negative associated with those words any longer. It's not a lie, it's how I feel. It's pure and honest and it never felt that way while we were a couple.
I know I love those words now, so I guess I just want you to know that a guy can change his minds about this. He may just need a lot of time to let his thinking work itself out. But if this is the only thing that's missing in your relationship, I'd say you're doing pretty well.