Some of the past unhappy endings when a companion leaves the series are real downers. I'd like the show a lot better and watch it more often if it was more upbeat.
I understand, but I disagree.
Part of the character of Doctor Who is her attempts to understand her place in the Universe. At this point, she is a demi-god with unique importance in the Universe, yet she still needs to deal with loss and with the consequences of her decisions.
That is an important part of the show for me.