You might feel familiar or important to each other from the start, and you both have the sense that you've come into each other's lives to help reconcile something from your pasts, or keep you on the same track you've been on for way too long now. In some sense, you're right. You were involved in a past life, and since you're coming together again in this one, it's to settle those old, unresolved issues.
In this life, your lover may have set out on a career or schooling path that's no longer interesting to them, but you take it upon yourself to be their taskmaster and to motivate them to stick to their chosen course. But people do change, and that course might not be the right one anymore, even if it once was. It would be a shame to prevent your partner from reaching out and grabbing hold of new opportunities, just because you think they ought to stick to the old plan. You try nevertheless, and you start to feel like you're overly responsible for them. They begin to look at you as a real downer -- an unwelcome, restrictive influence in their life. It really might be the best thing for them to simply switch tracks and try something new, but if you can't give them the space and flexibility they need to do this, they start to feel as if you are not supportive of them.