Many times when I hear about a couple's commitment issues, I hear there's difficulty in moving from one phase (perhaps dating) to the next phase (usually marriage). Or at times I'll hear that the commitment issue is about difficulty staying monogamous or cheating. These are actually two different issues that were lumped together and generalized as "commitment issues". So instead of working on the real issues, couples are caught up in fixing the wrong problem. In other words, if you have bad brakes and you only change your tires, you're still going to find yourself having trouble stopping. Many people have difficulty sticking to just one relationship or moving onto marriage in a relationship, and treating these two situations the same way is just scratching the surface. Although it may sound like therapy must be the only answer, it's not necessarily true. I'd like to share with you some tips in uncovering what may be underneath the "commitment" problem your partner may be displaying.
First, I would suggest looking at his/her other relationships. Ask yourself, does he/she have other relationships where he/she has been dedicated, committed, and followed through? Does he/she have any successful long term relationships, other than with you? This will help you determine whether the problem is in your particular relationship or something that is more personal to your partner. Secondly, I would suggest examining your own feelings and behaviors. Have you contributed to this problem by what you have been saying or how you've behaved with your partner? Have you put extra pressure on him/her unnecessarily? Lastly, I would suggest you both talk about what this relationship means to each of you. What would it mean to end or continue the relationship, and do you both have a common purpose in the relationship?
However, I will say as a disclaimer to ask these questions of your relationship only when you are willing to commit yourself to the answers.