I am also a bit confused towards Rey’s motivation.

I agree with Don in that Rey wants to belong somewhere. An expected reaction for someone so isolated. Further evidence for this:

She is in denial that her family will come in TFA
She saves BB8, her only friend possibly ever. Her insistence on getting BB8 to the resistance may be due to he trying to help the only friend she has even known.
She never outright helps the resistance. She only wields the lightsaber to save Finn, a friend.
She asks Luke to “show me her place in all this”.
It seems she only wants to know her parents after she is turned down by Luke. Luke’s grumpiness causes Rey to doubt the Jedi way and ponder about her parents once more (the cave).
She then develops a force connection with Ben and comes to believe he may have what she seeks (a place to belong to).

All this is building to Rey taking Ben’s hand.

Why would Rey want to join the Jedi? Luke has done nothing but criticize her. We all know the Jedi way is right, but from her POV she just met this grumpy hermit who doesn’t want her. Why should she care about the Jedi?

It seems Rey’s primary motivation in turning down Kylo is because she has friends in the resistance? Like Finn and BB8? But the entire film has been building up Kylo and Rey’s relationship. As for the “she realizes she does not need a purpose, she can make one herself” well wasn’t that where she was before TFA. On her own with no one?

I think Rey would have been a great character had she taken Ben’s hand and charted us a new, bold direction for TROS.

Now she feels more like a missed opportunity bordering “perfect” which is a shame given how close we were. I hope TROS does the best job they can with Rey- if they provide some clarification on her motivation I think the whole Mary Sue argument will finally be over.