Love is the first emotion that causes a desire for marriage. People want to share life with the person they love deeply.
And then what?
Shared life is a way to find new sources of happiness, new interests and create special experiences of companionate joy.
Many seek stability. Permanent partners give constant support to dreams and aspirations.
Of course, sexual pleasure is the most important reason for life together. The security and intimacy built by cohabitation become prerequisites for exploring eroticism. 
Every day new pleasure can be discovered and that is wonderful.
Permanent commitment is a journey through time with a willingness to explore feelings. Strength is given, love, passion, intimacy, companionship because the ultimate goal is the satisfaction of the expectation in happiness.
Enjoy!