People talk about relationships as simple or complicated.
Boy meets Girl (or vice versa) - they fall in love - they get married and live happily ever after. Simple enough, except that it rarely happens.
Boy meets Girl (or vice versa) - he is still involved with his ex. - but she decides to give it a go anyway - they argue - it becomes a habit which lasts even when he is no longer involved with his ex - she starts seeing someone because they are arguing ... This can get complicated.
These ways of talking tend to suggest that relationships, and the kind of relationships they are, are things which happen to people rather than things they make. It's true that people are told they need to "work at" their relationships, but they are only told this when the relationships seem to be going wrong. It is the stuff of parental advice and marital counselling: Try harder!
A different starting point would look on all relationships as things we make. Nothing just happens. Relationships are things of which we are capable, not things to which we are liable. (Author! Author! In this case, Rom Harré.)
Serious-minded people will make serious relationships and playful people will make playful relationships. Either way, they are working at their relationship from the very outset, in good times and bad.
Relationships work better when people understand that they need to be continuously made and re-made, as circumstances and the participants change. Habit is the background of everything we do, but habit cannot substitute for invention and re-invention.
We often describe relationships which last as "deep", but I would prefer to call them "elaborated". A great deal of detail has been filled in. There are the habits which form the background, but then there are the traditions, the rituals, the shared understandings - including sexual understandings - the attachments and affections, the trust, the humour, the projects and plans.
So a relationship is something people are always making. If they are lucky, they will be pleased with what they are creating and carry on, happy in what they are doing. If they are unlucky, one of them at least will conclude that it's time to tear up the paper and start over again. Either way, you don't rest on your laurels "till death us do part"