When you look at commitment-phobes closely, (and face it, many guys are) the root cause of this is the fear of unwanted consequences that may or may not come.

…The problem is, that is exactly what they bring by not committing.  They (unknowingly) exacerbate the problem by over-protecting themselves, destroying any chance of a decent relationship thinking they are the only one who fears the future.  

And if you ask them why they need a “guarantee” they will tell you they feel like they have more to lose by committing to you, as if they are the bigger prize?  

So what separates the “serial monogamous” from the losers?  

1. Their belief that the person they are with is the person they are supposed to be with.  This is based on principles, personality fit, respect, and shared goals/interests vs. mood(s), consecutive weeks of things not working out just right, and/or self-centeredness.
2. Adaptability.  These guys understand they will change over time and so will you…and they see that as a good thing because it probably means you will grow together.  Again, this is in stark contrast to the commitment-phobe who will only commit if you stay exactly the same.
3. Confidence.  Confidence means that if and when changes occur (and they will), they are man enough to handle whatever the future holds.  That is not to say they don’t desire a certain outcome per se, the difference is, they are good with whatever happens because they know you will be there with them.  Things don’t have to fit into a neat little box for the two of you to work, they work because you are together, and that is enough.