You don't see eye to eye when it comes to duties and responsibility in general. This is frustrating enough in realms that exist outside your relationship; if you have very different work ethics, for example, or senses of familial duty, or convictions about money and how much to save, how much to spend and on what, you find yourselves arguing your points again and again. Within the realm of your romance, any disagreement in this area is a real and immediate problem.
If you have very different ideas about how much emotional security you need, how much work should go into the relationship and so on, you both start to feel angry and even depressed, fast. What if one of you wants a serious commitment and the other doesn't? What if one of you wants to go to couples counseling and the other thinks that's the worst idea in the world? You come to a major standoff, at the very least. Arguments and bad feelings only increase if you can't find common ground in these areas, because the stakes are so high -- or at least, they feel extremely high. This doesn't have to mean the end of the relationship, but it does mean lots of communication and compromise are necessary. But who knows? Through your struggles with each other, you might just push each other and yourselves to come to an agreement or a commitment.