The ending of Allegiant, while heart-wrenching to the reader, was the perfect way to end the novel. Tris would never allow Caleb to die for her.
This epilogue allowed Tobias to move on with his life, and was written sensitively and realistically for these reasons:
1. There was a five year gap between Tris's death and this epilogue. It wasn't like the week after Tris died that Tobias completely forgot about her and kissed her best friend.
2. Christina SAVED Tobias by not allowing him to take the memory serum. If that isn't a good foundation for a relationship, then I don't know what is.
3. Christina is one of the few people in the world of Divergent that can understand Tobias because she also lost someone she loved.
4. As mentioned in the book, Tobias was eighteen when he fell in love with Tris. Their love cannot be invalidated, of course, but leaving him in a life where he grieves over his childhood love for as long as he may live is a cold-hearted thing to do.
So STOP hating on this book. If you don't appreciate the closure Veronica Roth gave Tobias, you haven't understood the Divergent series at all.