The foundation for any lasting relationship will include both trust and respect. When either of these are lost (or not really there to start with) the other will eventually evaporate.
It is quite possible to build a relationship out of mutual need. There are many combinations of needs that work, and there is nothing wrong in having this as one aspect of a relationship, but without trust and respect, this can break down to produce a toxic environment in which either or both "friends" suffer.
With trust and respect, true affection can develop (I'm not talking about lust here), along with admiration and a sense of caring that is at least somewhat unconditional. You quite simply want the best for the other person, even if there's "nothing in it" for you. When this is mutually expressed on a consistent enough basis, the relationship blossoms and life can take on a wonderful quality, as it deepens in meaning through having a real connection with another person.
In the Bible, the first epistle to the Corinthians, chapter 13, verses 4 to 7 sums it up by stating, "Love is patient, love is kind. It does not envy, it does not boast, it is not proud. It is not rude, it is not self-seeking, it is not easily angered, it keeps no record of wrongs. Love does not delight in evil but rejoices with the truth. It always protects, always trusts, always hopes, always perseveres."
In life, nothing is perfect, and so it also goes with relationships. For any relationship to survive the times when either or both people in the relationship notice some aspect that is short of their ideal, it is essential to have a realistic view and to accept that the other person is different, and that these differences are NOT something to be worked on in an effort to change them - or to complain about.
So acceptance is another essential element for any long term relationship to survive. Without it, sooner or later, the trust and respect is likely to break down, along with the relationship. It is through a lack of acceptance that people come to the opinion that they have "irreconcilable differences" with the other.
To this point, I have described relationships that do not have to include any sexual element. It may be between close friends, between a parent and a child (I am thinking more particularly of older children), or between brothers and sisters.