2. The Relationship's Already Formed
Now these two have history, a long confusing timey-wimey history. Obviously River already knew all of her adventures with the Doctor long before he did (I think?), so for her the relationship between them was already very strong. But over the series' where she popped up every now and then we saw them develop into a stronger, more together power couple, even with a cheeky snog, which the Doctor seemed very involved in. We've seen them face some of The Doctor's most terrifying foes and each time come out victorious but also a little more trusting and allied. River has already proved herself a worthy concubine by helping out The Doctor, even flying the TARDIS with much more finesse than he does himself. Most of all she's earned The Doctor's trust, which isn't easy. All in all if River came in as the companion around the time after the Angel's Take Manhattan (and her parents) with a solid relationship already in place I think these two would be a force to be reckoned with around the universe!