I've observed and learned that there are five important ways in which a man and woman must find compatibility, in order to form a successful long-term romantic relationship. While we all feel that there's no "perfect thing" and enter into a new romantic situation wondering what we can sacrifice, and what's non-negotiable, I would suggest that harmony on these five major points is essential.
Physical
Obvious, perhaps. But you must in fact find your partner both sexually appealing and beautiful. This means no need to change anything about them. Many discount sexual happiness, considering it a guilty pleasure or a perk, that they could do without if they are happy in many other "more important" ways. Sexual happiness creates the bliss and euphoria that glues you together with desire. Not touching each other, not giving each other that rapture and ecstasy turns a passionate love into cohabitation. It is what feeds and enforces the yin and yang energy of your unification.
You must also feel beautiful in your partner's eyes. Often again, we think there are things we would like to change about our mate, or things we can tolerate, but wish were different. These things are all subtly communicated in our action, inaction, words and silences. If there is something you find physically unattractive about your partner, your relationship will not work. I personally have been in situations where I felt judged, or merely "accepted". It erodes self esteem to the point of dejection.
Mental
You must be able to relate to each other intellectually. My mother once told me, "Never marry a man who is not as smart as you are." If you are unable to express openly and experience life verbally as well as physically, you will grow bored and disenchanted. You will seek stimulation elsewhere, finding yourself having animated conversations at cocktail parties with someone else and thinking: WOW, if only .... Now, this doesn't mean that an astrophysicist can only mate with another astrophysicist. But it is necessary to have someone of near equal intellect who you find fascinating and enlightened in their views and self-expression. If you can't do the New York Times crossword together, or compete in Scrabble, you will find your mind wandering off in search of more challenging puzzles.
This mental compatibility is also the basis for critical communication in a relationship. You *must* be able to talk to your partner. Whether it is about your fears and concerns, your problems, or your joys. You must be able to share these things, and often; and intellectual equality allows for that sense of understanding and clarity.
Emotional
It is debatable whether or not people must balance each other emotionally, or mirror each other. Generally speaking I find that balance works best. So if I tend to be anxious or worry about things, it is valuable that my partner be cool and un-flustered. Either way, it is critical that you understand and respect each other's emotional fabric, and this includes any abnormalities collected from childhood or other past experiences. Of course the best case scenario is two completely stable and emotionally healthy people who deal with all life's challenges and each other with humour and compassion. But life is not a text book, and more often we have our insecurities and quirks. Men in particular, generally hold women's generic emotionality in contempt. Women find men cold and un-invested. You must rather be able to empathize and value the strengths of each other's natures. See a woman as intuitive and emotionally open and giving. A man as pragmatic and refreshingly linear.
Spiritual
Your beliefs about life and death, and existence, if not the same or similar, must at least be mutually respected. Optimally they would be the same or similar, but it is possible for people to be fully joined in life without having identical beliefs about spirituality. Extreme polarizations would not work, however, such as atheism and devout Christianity. It would seem hard for a couple to love and respect each other, if one believed that the other was "going to Hell", while the other "prayed at the altar of sheepism" on Sunday. Furthermore, a faith which requires lifestyle changes and commitments which are not shared, would be very divisive as well. I recall an ex-boyfriend who was exploring Buddhism. Being an obsessive type, he was considering jumping into the deepest end with lessons and training, giving up meat and alcohol. A significant departure from our existing shared lifestyle at the time.
Social
You should and must have interests that are exclusively yours, that your partner is not involved in. It keeps the relationship fresh and intriguing, keeps you developing as individuals. It makes you continually interesting and new to each other. Having said that, the things that are most important to you must be shared. You passions and the things you love and enjoy most. Whether it's smoky blues bars or chic bistros. Hiking or travel. Do not think that you can be with someone who is afraid to fly, if your one true passion in life is seeing the world. It will not work. You will either be grounded and sad, or s/he will be home alone wondering where you are.
The five essential ingredients in the recipe for a happy relationship. When one of these needs is not being met, its importance becomes paramount. The way your tickle of hunger turns to a pang of starvation when you open the fridge and realise it's empty. Your need will not vanish, it will become even greater, and you will be forced to try and find a way to satisfy it outside of your relationship.
If you reflect back on past failed relationships, I wonder if you can identify the ways in which you were incompatible based on these axes. And I wonder if you, as I, knew from the beginning that you would have these issues, but hoped that they were manageable as "compromises". In relationships, as in all other aspects of life, there are needs and wants. I would suggest that these five are unforgiving needs, and the expense of any one will cost your complete, long-term happiness.