When I think about intimate relationships, for me the crux of it all boils down to this:
1) Am I willing to allow the other person to be exactly as they are, to accept and love them for their whole person, over the course of this relationship?
2) Am I willing to explore and express my whole person, to be true to myself over the course of this relationship?
3) Am I fully accepted and loved by the other person as I reveal more of who I am over the course of this relationship?
In the early days of a relationship it is very easy to naively say 'Yes' to these three questions. And it doesn't matter if the relationship started in your 20s, or 70s, or whether it is your 1st of 10th. It is always the same feeling of elation and optimism, and perhaps without it no relationship would ever start.
As time rolls on there will inevitably come more than one moment when your partner shows you part of who they are, and you don't like it. In fact you are repulsed by it. And correspondingly there will come a moment when you want to reveal part of who you are, and there is a risk that your partner will not accept it because it is not who they think you are or want you to be.
For me that is the true test of a relationship: that moment of seeing and accepting the other, and seeing and revealing ourselves.