Hi,
So, if the sentence 'The meaning of my life is she' were 'The meaning of my life is her' instead, does it sound natural to you? If not, why not?
No, it sounds a bit unnatural. 'The meaning of my life is Mary' sounds a little better. On the other hand, 'My problem is not you, it's her' sounds OK.
It's hard sometimes to explain why something sounds odd. I think one reason is that the high importance of what is being said, my statement about the meaning of my life, contrasts oddly with the use of the modest, humble little pronoun. It's like 'She's the most important aspect of my life but I won't bother to say her name'.
The whole choice of words seems a little unnatural to me. I'd be inclined to use the verb 'mean' in an active way, to give more life to what I'm saying. I'd also show that I have other meanings in my life by using a superlative. eg
Mary means more to me than anything else in my life.
Clive