I received this question from someone, and I think it’s such an interesting topic that I decided to post about it.
I have observed and thought about this many times. In the context of a significant relationship, when a person’s planets, especially their Sun, Moon, or Ascendant, fall or overlay into your 7th house (the part of the chart representing partnership), yes it does mean that they embody in a big way whatever it is that you need in a partner, so you may feel strongly drawn to them. They balance you and complete you in a sense. This can apply to a romantic, friendship, business, or any other significant partnership.
So how does this affect the person whose planets or Ascendant fall into your 7th house? In my experience, it is not the same dynamic, but the connection and attraction is still mutual. The person whose planets or Ascendant fall into your 7th house feels called and inspired to be your partner. It feels natural to them to be what you want and need in a partner. This happens on a relatively subliminal level. Some authors say the attraction is stronger for the person whose 7th house is being over-layed by the partner’s planets, as opposed to the partner, but I have not found that to be the case. Your 7th house (i.e. the partnership) provides for them an area and a context for vital expression of part of themselves.