Within the context of a romantic relationship, friendship.

No idea. I kept my last one going for 12 years by being a doormat and workhorse. Not doing that again.

Time.

Communication, Unconditional Trust, Mutual respect and at least some interest in common.

accepting the suffering that is the toll for giving and receiving love.

No idea.

Respect, chemistry, shared physical connections.

love, trust, sex, commitment, respect.

I think this differs with every relationship. Some factors for me would include (in no particular order):

A certain level of compatability in several areas
Physical
Some similar interests
Similar philosophy
Non-believer (I think being with a X-tian could work now that my kids are fully formed, but I'd still favor a non-believer)
Compromise (there has to be some)
Honesty
Communication

Human realtionships are complicated things, IMHO. This just scratches the surface. LOL!

First, emotional stability for both is critical. Commonalities (mental, emotional and physical) are also very important but, in the end, it takes work on both sides. If one is trying and the other not it will not work.

I have been through failed intimate and friendly relationships. I have also been ones that have lasted. I hope I have learned something from both.

Common interests,trust caring the same thing that make good allies . These are the things trumpie is trying to destroy with our allies and friends. He treats them like crap and then ask for their support. He must be a pure genius?

Determination. Working through problems with respect. Choosing them over and over again.

You asked two questions. What makes a relationship last and what makes them strong long-term? Long lasting relationships happen for many reasons, children, fear of being alone, financial reasons, fear of starting over, and some I'm sure I can't think of. I believe strong long-term relationships are based on being best friends.

It came after the one before.

Great friendship and humor are important.

Complete honesty and mutual respect for each other.

When I figure that out, I can disseminate that to the world. Otherwise, honesty, mutual trust, mutual respect, common ground, chemstry, communication, have always put me into the friendzone. In my opinion, that's the one to put your love into.

Honesty, respect, mutual dovetail desires

I don't think there is a golden rule. I have known several couples married for over 50 years and they have different reasons for the longevity. I also know a couple that was married three times and divorced twice. They had a set of ideas that were unique to themselves.

Understanding and choosing wisely in the first place

Duct tape.

Being that I have yet to have a successful long term relationship, I can only guess. But from family members who have been married 40+ years, I think the one thing that they all have told me is communication is the real key. That and a recognition that the two of you will not always agree.

Trust, communication, and enjoying things together.

If I had the answer to that i would not have been single for the last 5 or so years. Looking at my grandparents think to be tollerent is some of the key.

hard work, commitment, love, respect, communication