Support Through Life
Long term relationships give people a partner to support them through the difficult times of life, and they often come to depend upon that person. Being able to support another person and receive it in return makes life's burdens easier, and partners understand how valuable this is in their life. Few people would trade their partner for anyone, and it is the knowledge of this support that keeps them going when they disagree or argue. Being there for each other is the reason they made the commitment in the first place.
A partner who is unsupportive will not last long in a serious relationship, but this is how it should be. Dependence on each other is one of the most important reasons to stay in a relationship, and getting through life together is the whole point. Turning to a partner in good times and bad makes the relationship worth all the struggles and compromises necessary to make it work.