Someone asked the following conundrum. This was asked in the context of whether it's "shallow" for a person to refuse another as a partner, based solely on that they aren't sexually compatible:
"If your love is predicated on sex then is it really love or is it just two people using each other?"
Loving someone, and being useful to them, are not opposites. The two work together. To love someone is to offer yourself to be useful to them. It is to serve them gladly, with the expectation that this will be appreciated and returned. To accept being loved is to welcome this offer; to return it, and appreciate it.
Love is a willingness to serve: without coercion, and without feeling coerced.
"Love" and "relationship", though, are different things. Every one of us can love everyone, hypothetically. However, we can't have functional relationships with people who can't meet our needs.
Relationships are love + function. If you take away the function, the love remains. However, without function, love alone is not enough for a relationship.
This is why relationships based on compatibility can work. We can love everyone if there's no reason against it. So when two people are complementary, there's no reason for love not to arise. But the reverse is not true: two people who feel deep and passionate love for each other can simply not be compatible.