Some sort of struggle between you, subtle or overt, pits your lover's true self against your path in life. Somehow, you two have needs or aims that are at odds with one another. This influence isn't apparent at the beginning of your relationship; its negative effects could be an unhappy surprise later down the line.
You could start off feeling perfectly in tandem -- trusting of one another, sharing goals and ideals. Over time, though, it becomes more apparent that you're following two very different paths, ones that may even work at cross-purposes to each other. You find that you simply have different views of the world, or different ways of approaching obstacles and challenges. Your lover could feel dragged down by what is perceived as your pessimism or weakness, or they could feel as if your aims in life are in direct conflict with their own. What's more likely is that it's all a matter of perception -- that what looks like weakness is simply a different personality from theirs; or what feels frustrating in your life choices is actually those choices serving as a frame of reference for what your partner doesn't want to do in their life. You could stand to learn from one another, but struggle and frustration are more likely. Eventually, you both have to decide whether the relationship is worth carrying on.