According to Y Combinator co-founder Paul Graham, what is one thing that often surprises newbie entrepreneurs about the start-up journey? Just how much their relationship with their co-founder comes to resemble a marriage.

Here's how one founder of a successful start-up put it to Graham (in slightly off color language): "One thing that surprised me is how the relationship of startup founders goes from a friendship to a marriage. My relationship with my co-founder went from just being friends to seeing each other all the time, fretting over the finances and cleaning up s**t. And the startup was our baby. I summed it up once like this: 'It's like we're married, but we're not f***ing.'"

Graham's advice based on this reality is simple and sensible -- choose your co-founders wisely. But perhaps there's another possible takeaway: if you want a long and productive relationship with your co-founder, maybe you should handle your business partner much the same way you handle your spouse.

That's the advice of entrepreneur Feifan Wang. In a thought-provoking blog post recently, he explains that while listening to a friend talk about sustaining his happy marriage, it occurred to him that much of the same behaviors and rules of thumb that apply to contented romantic partners also apply to successful co-founders. In the post he shares several of these bits of cross-over wisdom, including the two tips below.

Give 60 percent

In any sort of relationship, you don't want to give less than your fair share, but you also don't want to give so much more that you end up resentful. How do you walk this delicate balance between selfishness and aggrieved martyrdom? Wang suggests "the 60 percent rule."

"Always commit to giving an average of 60 percent. Such a mentality puts you in a mode where you are always giving a little more than your share, but not too much that you are doing all the work. It's a delicate balance because it's easy for the other person to take you for granted. But, you leave your partner enough room to express his/her work and contribute to the business. It also builds trust between both parties since the other person is always trying to give more," he explains.

Don't keep score

No matter how healthy a co-founder relationship, small irritations and perceived slights are bound to come up. What's the right response when they do? Definitely not taking a mental note of the issue for later use. Wang calls this mental tabulation of complaints and imbalances, "keeping score." It's unhealthy in both business and romance, he insists and urges entrepreneurs use this alternate approach instead:

"The key to not keeping score is to bring it up the moment it makes you feel bad. Make it clear to your partner on how his/her actions make you feel and how you would like them to improve. Come from a place of reason, and try not to let your emotions hijack your behavior. Doing so helps you squash any difficult feelings immediately and gives the other person an opportunity to respond and improve."

Are there any other words of wisdom on marriage that can also apply to co-founders?