What happens alot is that one partner finds him or herself feeling committed but doesn't sense the same level of commitment from the other person.
Getting someone who doesn't seem ready to commit or resists commitment to commit is probably one of the most difficult things to do. Just as you can't insist that someone love you or love you back, you can't insist that someone commits to you.
What do you do if you are ready for a commitment and the other person doesn't seem quite there yet?
Do you just wait for him or her to commit to you -- and to the relationship?
It's important to remember that while resistance to commitment can sometimes seem like your world has come to a standstill, resistance to commitment can also be healthy. It can act like brakes slowing it down and allowing you to see things more clearly (is this person right for you) and also manage the process at the pace that is comfortable to you emotionally and financially.
With that said, it's not a very good idea not to wait too long before bringing up commitment.
Things might just get too comfortable to change...