The first Doctor had two companions die horribly, in a row, in quick succession. This really upset Steven, who was there for all of it. Steven is grieving. Steven is, possibly, blaming him a little bit.
When faced with the prospect of adopting a new companion, he won't. Traveling with him could just get her killed, and Steven will be terribly upset about that. If she dies off his watch, that's not his fault and not his responsibility. Steven can't blame him for that. This is where she belongs, therefore it is best to leave her here, no matter what may happen. He hasn't stolen her. Stealing people away only to get them killed far from home is wrong. See, he's finally learned that! He is not doing it this time. He has such good self restraint! Steven will be proud of him for this. Steven will be pleased that he is being responsible.
But Steven is angry.
It doesn't make sense, he's doing the Right Thing! It's very confusing.
Better take the next girl to come along. Now Steven is happy. Humans are so inconsistent and impossible to understand!
