Although you might have gotten along wonderfully at first, and felt as if there's plenty of commonality and good feeling between you, you might feel less and less so as time goes on. If you are past the early stages together, you start to notice a strange sense of dissatisfaction stemming from the fact that your life paths are somewhat in conflict. What your lover needs to feel comfortable in life, as well as in the relationship, might be the exact opposite of what's good for you.
Look at it like this: if you must travel a lot for your career, but they need lots of together-time and emotional reassurance to feel stable in the relationship, those two needs are equally valid but in direct conflict. That's the kind of struggle you two encounter with each other. It is your partner who likely ends up feeling frustrated at the fact that you are unaware of their needs, or unable to accommodate them. It might even seem like you just don't care about their needs in the face of your own, which could be true, to an extent; we must each look out for ourselves first, after all. But doing so doesn't have to mean that you neglect the needs of your partner. If you two want to make this last, you should be aware that it takes compromise on both sides. You have to step outside of what's most comfortable for you in order to accommodate your lover's needs as well as your own.