I've always assumed that this happens to everyone. It certainly happens to me. Some days I can look at my dearest friends and there's just a void. Nothing bad. Just nothing.

For me the void always goes away. It's a phase in the relationship. Hopefully that's all that's happening to you. If it doesn't change (it sounds like this has been going on for awhile), you may want to consider that either something is up with you (are you detached with a lot of things lately?) or that maybe your feelings for this person have legitimately changed. If the later is the case, you just need to adjust your thinking and let the relationship be what it is.

My two cents.

Cyran0