3. River Gave Her Regenerations & Life For The Doctor
Let's not forget that the first time we saw River in 'The Library' she gave her life so the Doctor could live on. This was obviously when everyone still assumed she was human and didn't realise that once upon a time she may have regenerated herself. However you then flash forward three series and one Doctor and we see River already gave away all her regenerations to save him from his demise once before, granted this was after she herself gave The Doctor the kiss of death with her lethal lipstick. We've seen time and time again that River is willing to put her life in The Doctor's hands and trusts him implicitly. That's got to be an A* for loyalty and that's what the Doctor needs in a companion.