The idealism that you may bring to the relationship is drained through your interactions with this person, and it's hard to figure out why this is happening. You might not even notice the effects, while they are only dimly aware that something is just off between you. The problem is that you both try too hard to be involved in each other's emotional lives, and you fail to set up boundaries.
Your energies are drained as a result, especially if you tend to argue a lot. You're overly sensitive to this person on an emotional level; you sense it when they're feeling down or hurt, and you take the responsibility for those feelings onto yourself. You might even start beating yourself up emotionally for causing the problem. This isn't a good idea, obviously. Any emotional disconnect or oversensitivity isn't the fault of either of you; it's just an unavoidable fact of this particular pairing. Trying to bridge the emotional distance could prove difficult, or even futile. It might be a better idea just to stop trying. If you can accept that there's a level of emotional disconnect and discomfort between you and just move on, you might find that it's not such a problem as you had thought it was.