So for ultimate success, the following three categories must be met in order (Especially the first 2, these two are essential):
1) Feelings for each other.
This goes without saying, no matter how hard a guy tries, unless there is true feelings of love between the two people, the relationship may proceed but will be doomed to failure.
2) One party (guy or girl - depending on who chased who I guess) who has a very strong understanding of the strengths and weaknesses of them self AND the opposite sex.
To explain, I'll take create an example, I'll be the guy. We will also be working under the hypothesis that criterion 1 has been met as well. So what this means is that I must have a strong and deep understanding of the good points of the girl and her flaws. Moreover, I need to understand what my own good points are and what my flaws are. This way, if anything the girl does ever touch my flaw, I'll be able to sidestep it and avoid any major confrontation. Likewise, I'll be able avoid inflaming the girl if I understand what her flaws are. Furthermore, I'll be able to utilise my strengths to address the want and needs of the girl.
Of course at this point, you might be wondering something and that is one party may not be sufficient for ultimate success and I totally agree with your point. Deep understanding from one part does only so much and may wear out that person. But, if the feelings exist, the side that lacks the understanding may eventually develop this skill and therefore lead to success in long term. For the sake of this post, I'll just use this "One Party Criterion".
This reminds me of a famous quote in the book "The Art of War": IF you know yourself and your enemy, for every hundred battles fought, you will WIN every hundred!
Likewise, it's common knowledge for relationships to succeed, both parties must contribute 100%: I am not reiterating this, I am just trying to state the more specifics.
Now, there is many other factors out there that may influence the success of any relationship but these two are the critical ones. However, I will list the some I feel that is important:
3) The X-Factors
Religion - Sometimes religions clash but I don't think this is true in a majority of the cases. This is not a major impediment.
Family Values - This is a big one. The power of families are recognised throughout the world particularly in conservative cultures. Some families want "good kids" for their son or daughter, others don't want certain races. Some want them to same religion. Basically, if the family values clash with certain traits of either party, it can cause trouble. But in 21st century, we understand marriage is a person's biggest thing in their life so families tend to involve themself less and less unless the guy/girl is a hopeless person with no charm or future prospects...
Personalities - According to Myer and Briggs, there is 16 types of personalities and each often have their own strengths and weaknesses. This goes hand in hand with criterion 2 listed previously - a lack of understanding about your partner's personality will lead to failure. If you know whether their personality suit you, then you are more likely to keep it going or cut it off. Any incorrect choice will fuck (pardon the language) you up.
Intellect - It's simple. Either the girl can't keep up or the guy can't keep up. One side gets bored and fed up and just go find someone more interesting and someone who can keep up.
Inability to Commit (Goes hand in hand with lack of loyalty) - This is hard to explain as to why. People I've interviewed are simply unable to answer this question. Guys and girls just seem to see it as something deeply inherent. I feel this is something to do with feelings. Putting it simply, criterion 1 (feelings) are just not deep enough and the attraction is simply infatuation, not love.
Loss of Interest - This can happen due to work, friends and family commitments. Distractions lead one and another to other directions and someone else.
Poor Communication - Goes without saying.
Trust & Honesty - As suggested by a few, these two must be in place. Jealousy is a curse they say and lies will be the end of you.
OK, let's wrap it up for now. I hope you understand what I mean and that for any relationship to work, the top 2 criteria (feelings of love & deep understanding by at least one party of another).
The X-Factors will be external and internal forces that will impact upon the two criteria. The following diagram illustrates what I mean.
Sorry for the drawing haha. It's late so I'll leave it to everyone to ponder what I've written.
Please leave a comment on whether you think I'm right or wrong and more importantly why. Your comments will be valuable for my future research into the enigma of woman!