Planets conjunct any angles in synastry are always deeply felt by the house person (except maybe when it comes to the ASC).While the MC isn't considered a "romantic" or even relationship-oriented house, it's still a very personal point in any chart, and points to what we'd like to achieve in life, what we'd like to be or become, the traits we find admirable and may want to develop in ourselves, and so on.
So if someone's planet sits on your MC, you may idealise that planet and the characteristics that person has in relation to that planet.
They may embody that exact thing you think is respectable and admirable.
With Mars it could be that you like the way they do things, the way they express their drive / passion, and more generally their body and physical energy (Mars rules Aries and the 1st house - the physical Self).
Just want to add that sometimes it's not even conscious, you may not find yourself actually "admiring" them per se, but you can sense a sort of pull towards that person on an energetic level.