In our current modern cultural environment, longterm relationships (whether co-habitating or married) are most often a main source for friendship (and we all know that PD27 says: "Of all the things which wisdom acquires to produce the blessedness of the complete life, far the greatest is the possession of friendship.")
But, what does a healthy relationship look like? Of course you could say that a healthy relationship is also a pleasurable relationship. I just found this very good article which gives very good insights into this:
The following are the 8 points from the article, and the link to the full article follows below:
Taking interest
Acceptance and respect
Positive regard
Meeting basic needs
Positive interactions
Solve problems
Rupture and repair
Reciprocity