What is the one single most important ingredient for a long & happy marriage?

i think the single most important ingredient for a long and happy marriage is LOVE.because when you have it,everything will follow,patience,respect,trust and etc. many old couple stay happily with their marriage because they have love,if they don't have it,there's no reason for them to stay,not even their kids.

There are many factors that makes a longer lasting happiness to both husband and wife. There is trust, honesty, caress, love and friendship. In friendship, there is always limitations. For me, if both husband and wife maintain its limitations towards its others rights and privileges, then one has no reason to separate...Love of course should always be there. Because if there's none, then what's the reason of living together as husband and wife?, both can remain friends. Trust should never be forgotten in marriage, because there are always trials and temptations that come along. Trust should be there to serve as pillars to what is called marriage. Honesty, should be possessed by both, to give reason why both husband and wife should trust each other. Caress is the action of Love. At all times, both husband and wife has the reason to go home because each knew that either is waiting to care for him or her at home... Its not only single factor(for a long and happy marriage)but many...

i think te one thing to sustain a relationship is RESPECT. when you respect a person then you automatically love, trust and try to understand the person.

i think love is the first ingredients then followed by honesty/ caring/ trust/ and the best not just a hubby and wife but the couple most be bestfriends too....like a lover and a bestfriend at the same time........

There are number of things which combinedly forms the sensation of happiness. But as you have asked about the most important single factor of being happy in this case, i will reply trust. Some may have different opinions here, but i will give priority to this single factor, mutual trust. Thank you.