Even if things started off on a great foot -- getting along well, there's a definite spark of attraction running between you, heating things up -- as you get to know each other better, a wall springs up between you, cooling things down considerably. When a relationship is running along on a positive track, both people involved feel wonderful -- energized, optimistic, full of plans and hopes for the future. It's a big difference between that scenario and one in which a couple is simply lacking affection for and attraction to each other, for no apparent reason.
The latter is likely the situation between the two of you, and it makes it very hard to get to the point of making a real commitment to each other because you don't feel secure in the relationship. Even if your lover gives you no reason not to trust them, there are still problems with jealousy and possessiveness between you. When there's a wall like this up between two people, insecurities spring up, jealousy kicks in and then you're in a fine mess. Instead of allowing that to happen, why not take a more proactive approach? Talk things out as directly as you can. If you find that the spark between you is just gone, it may be best to just move on and find someone with whom you're more compatible.