We don't see how they cope with the realities of life. We don't see how they adjust to living together or what happens after the wedding and the honeymoon. Most of all, we don't see how couples stay together, year after year, through raising kids, job losses, ill health, in laws and all the other daily things that make up a life.
"Deal breakers" are those behaviors or conditions that one partner is unable or unwilling to tolerate in a relationship. Because "tolerance" is a relative term and subject to everyone's unique capacity to accept varying degrees of distress or discomfort, there is no generic, one-size-fits-all answer to this question.