If your partner does not care for you then you are not married and hence by definition marriage is a good state to achieve (as you will be with someone who cares for you). How to achieve a good marriage is another question (dealt with elsewhere), but the rationale for developing a bond of caring for another person is as follows:

•  you need someone who will care for you in later life and in case of difficulty

•  in a good marriage you develop a bond stronger than true friendship such that you are assured that your partner will care for you in such circumstances

•  outside of marriage, it is not possible to get such care or valid promises

•  in a good marriage, you are blissfully happy, peacefully ecstatic with your partner. It is not possible to get such a state outside of marriage

•  in everyday life there are tensions and stresses. Sexual activity and family life generally are ideally suited to release these tensions

•  outside of marriage you are in your heart, if you stop and listen, not really happy

•  being without a marital partner creates its own stresses, strains and worries besides not allowing other strains to be released.