In all things you must present a combined front to the world. Whatever your independent and individual beliefs, marriage is a uniting of the spirit as much as it is of the flesh. If you have no spiritual belief in and bond to your marriage, you will be lacking that which sets us aside from the beasts of the field. The second is, indeed, like unto it. Put everyone and everything else second to your marriage.
Children come, grow and go.
Parents pass on. Siblings and other relatives scatter. Friends relocate or you may grow beyond them, or them beyond you. Jobs and the people in them change. In the end, all you have to totally rely on is yourselves, and all that within the framework of your marriage.