I can only speak from personal experience. I've been married to two men, both of whom were polar opposites of me in everything that matters for the long term--personality, character, temperament, values, interests, etc. I divorced them both after unhappy marriages that couldn't be "fixed." Need I say more?
Actually, I will. It's obvious my only "credentials" for giving marital advice originate from the lessons I learned the hard way. My insight was gained by hindsight, but I consider it valuable because of the anguish through which I earned it.
I urge anyone contemplating marriage to realize that this person to whom you wish to say "I do" should, above all else, be someone you would choose for a best friend...someone with whom you enjoy spending time and doing things you BOTH enjoy, and with whom you can carry on a truly interesting conversation. Make that MANY interesting conversations. You should also want happiness for your partner as well as yourself. Love is not selfish, but neither should it be totally selfless (for therein lies eventual resentment).
Marriage should NOT be based on infatuation, physical attraction (alone), a person's outward looks and charms, financial wherewithal, social status or any of the many other reasons people marry. After all, if you hope to stay with your spouse for life (and I assume most people--though not all--begin marriage with that intent), there should be something stable and real to bind the two of you together after that first emotional and physical heat burns off. (And it will, which is not to say that romance and passion must or will end--they shouldn't. However, those people who expect an ongoing state-of-bliss honeymoon feeling to last forever are the ones who later have affairs, searching for that illusive feeling again like a drug fix. It's unrealistic.)
A poorly matched couple should not expect having children together to suddenly give them enough in common to make up for their differences. Children often get caught in the crossfire between battling parents, and it's a mistake to think having a baby will bolster a weak marriage.
I can't imagine anything more wonderful than being married to one's best friend, and this is what I would wish for anyone contemplating the state of matrimony.