There are certain qualities we look for in sexual partners, and other things that we look for in romantic partners, and still other things that we look for in friendships. And the person you love should be a romantic partner, a sexual partner, and a good friend all rolled into one. But if our expectations for these three roles are drastically different, it seems unfair to ask one person to try and act like three. This is part of why there is so much infidelity, unhappiness, divorce, and lonliness. We have unrealistic expectations. We expect societal ideals that can't plausibly coexist in the same person.
For instance, a man might look for exciting, adventurous sexual partners-- women who challenge and stimulate him, women who won't be tied down. Then when he marries one, he starts expecting her to suddenly become a conservative housewife. Not gonna happen. You can try and force it for a while, and on the surface it might seem to work, but it will be soul-killing, and eventually the relationship will end.
Or say a woman seeks out strong, confident, assertive men, men who show no fear, who are who they are with no apologies. They attract her. They make her feel wanted. They are men of action. But then the woman starts wanting this same man to be a sensitive guy for her, to talk about feelings, to gossip about so-and-so. That's just not who he is though; if he cares enough he'll give it a go, but she's trying to make him be two people at once for her. He'll either fail outright, or succeed for a while and then snap.
To avoid conflicting expectations, I think it best to have certain valued qualities that are at the root of all your relationships. What do you care about? No, do not count the qualities that would be nice. Do not include the qualties you think you want. Scrap all that. Scraps the minute details. Scraps wants. What do you NEED? What values are essential to you? What universally qualifies a person as worthwhile in your book? Use those qualifiers and let the other expectations go. If you rely only on these qualifiers (which should be relatively simple and few in number), then you'll know when you like a person that he or she is a candidate as a friend and a romantic partner and a sexual partner-- because you use the same standard of measurement for each.
Example of a universal qualifier: Respect. Every type of relationship is enhanced and deepened by the presence of respect. If you don't respect someone, you really shouldn't date them. It's not gonna end pretty.
No one is perfect. It is impossible for even your soulmate, your life's true love, to be "your everything." No single person can be everything for you because that would involve possessing opposing qualities that cannot coexist in one person. Example: Let's say you like to gossip, but you also have secrets you'd like kept. It is unfair to expect your spouse, significant other, or any one person to be both a gossip buddy and a secret keeper.
It is important to have realistic expectations of people, to know what they're good at and what they're not. It is also important to have a variety of people in your life to fulfill your different needs. The people you get closest to will be able to satisfy several of your needs, many more than those more distant. But no one can satisfy them all.