Everyone is disappointed in one way or another with other people. Understanding why that is, is crucial to happiness and contentment.
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Let's consider who your ideal friend or partner would be. Would it not be someone who agrees with you most, does things the way you like them done, is nicest to you, follows your schedule best, looks out after your interests with constant diligence, respects, defers to, forgives, trusts, and understands you?
 
In other words, someone who does not disappoint you would be someone who puts you first in their lives.
 
Obviously, nobody like that exists, with one exception, you yourself. You are your own best friend and the least likely to disappoint. That's easy enough to understand. What is more difficult to come to grips with is the fact that everyone else is their own best friend as well.
 
Everyone puts their own interests above everyone else's. This blissfull addiction to ourselves is not a pathology. It's a biological imperative, a key to survival. In our natural setting—what we are designed for—staying alive meant tending to one's own needs. We are hard wired to think of ourselves first.
 
This is not to say there are not giving and selfless—to a degree—people and acts. But no saint ever lived their life, moment by moment, thinking of and fulfilling the needs of another person. Even ostensibly compassionate and philanthropic professions and organizations are betrayed by their fees and salaries. In fact, some of the highest incomes are reported by doctors, lawyers, and administrators in "non-profit" organizations.
 
As cynical as it may sound, the reality is that selflessness can be thought of as a moral cloak cast over selfish motives. Even giving our life for another is selfish to a degree. Whatever we do is done because it brings us personally the most pleasure, relief from guilt, satisfaction for obedience, or duty from love. No human act can be removed from selfishness.
 
The take away from this is that everyone is going to disappoint you to one degree or another. They have their own best interests to tend to; you have yours. They have their life; you have yours.
 
If this is not immediately apparent, think about just one aspect of social interaction, conversation. Note how the other person speaks mostly about themselves and their life. While they speak, you are thinking about what you can say about yourself and your life. It's all you can do to hold back from interrupting with your own personal litany. If you quietly, attentively, and politely listen, you are considered by them to be a great conversationalist.
 
What creates the notion that another person should spend their lives thinking about us and fulfilling our needs is our childhood. That is a very unique and singular time when other people—parents— do tend to us and even put our needs above theirs.
 
Unfortunately there is no official graduation from childhood. Nor do any of us fully make the transition. Although we want all the independence and perks of adulthood, each of us, to one degree or another, long for the safety, security, dependence, and love that a child wants and needs.
 
But what we want, and what the reality of the world of adults is, are two different things.
 
Being an adult, by definition, means not being a child. It means being self sufficient and not expecting another person, a surrogate mommy or daddy, to meet our every need.
 
If we carry the child-like expectations into marriage or other relationships, we will be doomed to constant disappointment.
 
Moreover, egoism and narcissism, i.e., the assumption that others are on Earth to serve our needs, places a cruel burden on those who allow themselves to be entrapped by such constant expectations. People who spend days racing after everyone else's happiness, attempting to be everything to everyone, denying the need for self, ultimately become resentful, if not hostile. As in all things of life, balance is key.
 
Being an adult means facing reality, not illusions and wishful expectations of others. Yes we can have wonderful relationships, share common interests with another, and even do with or for another that which does not fit our personal desires. Healthy relationships require give and take. A hermit can be me me me, but a social person must recognize that everyone is their own best friend and requires space for their own needs as well as recognition and love from others.
 
Keeping the reality that everyone has their own best interests at heart, is key to understanding yourself, the actions of others, and what is required to have pleasant relationships without conflict and unrealistic expectations of yourself or others.