A Saturn conjunction to a personal planet in the other partner's chart is the first thing that comes to mind. Sun, Moon or Venus would be the strongest.

I agree with Saturn. 

I have to add Pluto/Venus aspects… it's very hard to let go of a connection that is so deep and consuming. 

Nodal aspects… strong nodal aspects, conjunctions, can feel very sticky and binding. It won't necessarily last, but it's usually significant enough to last at least for a while.

Interesting. My husband and I have moon/Saturn opposition. I do think we are there for each other emotionally on a very serious level. From day 1.  Which I always thought would be more of a downer astrologically, but we definitely don’t bring each other down. So that’s another way to look at it. 

All my longterm relationships had someone's Saturn conjunct Asc/Desc axis for me. Also Sun/Moon conj.

I think Saturn is quite misunderstood. It gets painted with such a negative, depressing brush. I don't think you can have a long-term relationship without a major Saturn aspect. It rules binding commitments, time, and karma… most serious long-term relationships require these. 

Too much Saturn and not enough Jupiter or Venus… okay, it's like going to school without having any recess or lunch break, understandably difficult. But otherwise, necessary.