After a while, you feel more and more out of sync with each other, and you might realize it has to do with security -- the stability of your relationship, your finances or anything else that might come up. It might not be completely apparent at first, but over time you both start to realize that you don't really match up when it comes to questions of responsibility and duty. You might have really different attitudes about money and family, for example, or you might find that a commitment is important to only one of you -- or you mean different things when you talk about romantic commitment.
If your communication is already afflicted, this may be the most difficult part of this aspect -- you have a hard time figuring out what your partner's position is on these various subjects, because you just don't understand where they're coming from! Little disagreements start to erode the sense of trust between you, and you could end up feeling like you just can't rely on each other to be what you consider to be a decent person and romantic partner. It helps, first off, to recognize and admit that you're compatible on this level. Beyond that, it helps even more to remember that the differences between you don't make either of you bad people. They simply make you who you are. Are you able to accept each other despite your differences?