It is a transaction based on perceived value. The value sought by one person may not be the same value sought by another. What value does the other person bring to the relationship?
It doesn't have to be looks, and it doesn't have to be wealth. People should choose wisely though because things such as wealth and beauty can change very quickly.
I recommend people find someone who has similar core values for that reason. Those things should not easily or quickly change. If someone believes in furthering the same cause in life as you, they are likely to be loyal to you forever.
Take for example furthering liberty. That's not something a person will wake up one day and say, "Nah, eff that. I'm not doing that anymore." If you commit to a person who believes in the same things, it is a pretty sure bet.