What (holds) a relationship together is both people deciding that no matter what happens in their relationship they're not going to walk away! This can be unhealthy or lead to a life of unhappiness.
We make the assumption that only (happy) couples stay together.
However it' not uncommon to learn of someone who has stayed in an abusive relationship for 20 years, or with someone who cheats frequently on them, has a domineering controlling personality, jealous/possessive, withholds affection and sex, lives in separate areas of the house, one person is raging alcoholic/drug addict or completely financially irresponsible.
If being "happy" is not an important requirement for one to stay in a relationship then it's easy to simply take what comes.
On the other hand if you do have "deal breakers", believe in standing up for yourself, have boundaries, expectations, and certain requirements then it can be a real challenge to stay in any relationship when you're unhappy/not getting what you want/need.
"Unconditional love" in relationships is a myth for most of us. In fact we're told it's healthy for us to have "deal breakers" in order to maintain our own self-esteem and love of self. I suppose the question everyone really wants the answer to is;
How do couples remain happy in long-term relationships?