It would be fantastic if it could all work out as beautiful and exiting as romance novels....but reality requires a great deal more hard work.

Nothing can be as complicated as the why's and wherefores of realtionships, but one of the culprits of major disappointments and misunderstandings comes from the fact that people tend to show their best side in the beginning. This is probably not an intentional or mean spirited desire to deceive, in most cases. Rather hormones and brain chemicals play a big part in the exitement we feel when we first meet someone we are attracted to. Over time, as a couple encounters various challenges, each individual in the team will draw from a mixed bag of experiences, background, etc., and draw conclusions from that as to how the partner 'ought' to behave/respond. When the partner doesn't follow suit, then the other begins to complain and demand in a desperate attempt to change the other and secure 'happiness'.
Is this unreasonable, unfair? That is as huge a question as there are people on the planet. In some cases it might be totally unfair, while in others, real intolerable abuse may lead to at least some healthy coomplaining. But obviously, there are far too many of us on earht to make any hard rules about what is right and wrong to fit every situation. Things must be weighed in every unique relationship.
However, there is a solution to the overwhelming majority of disappointments and unmet expectations. Getting to know another person without hurry, giving the possibility of real unity to develop over time, without forcing it. Waiting until experience upon experience allows each potential partner to see how the other person responds in a wide array of mixed circumstances. Some people jump in bed the first night they meet someone, and then worry about whether this person is the right partner in life or not. That's definitely putting the cart before the horse and rarely will this system ever develop into true lasting companionship.
Like everything else, the business of living a rewarding life requires lots of work, planning and patience. Nothing, but nothing can ever be taken for granted, but the reward of thinking and planning wisely is peace of mind. But something else is even more important than trying to find hapiness from another person - that never works! First a person needs to find happiness within, and when that's truly achieved, they will move on to attract others of like mind and self confidence. A truly confident person will find things to be joyful all throughout life, whether the seasons offer someone else to share that contentment with, or not.