The great American humorist Mark Twain certainly spoke for me when he said, "The human race is a race of cowards; and I am not only marching in that procession, but carrying a banner." He certainly spoke for me - and for billions of others. However, it's important to distinguish between caution and cowardice, and I suspect that while Twain may have had the word "caution" in mind when he uttered that remark, he knew it wasn't a quotable moment if he started the quote by saying "The human race is a race of cautious individuals."
Natural selection rewards the cautious. At the slightest hint of danger, animals scurry for safety - only in human beings (and possibly the higher primates, I don't know) does curiosity overcome caution to any great extent. Another aspect of the animal kingdom with which I am unfamiliar is the extent to which rational decision-making plays a part. However, human beings have the ability to make rational decisions, and these decision often involve alternatives in which there are different degrees of risk involved. These decisions occur in all areas of human endeavor - personal relationships, business, and the quest for self-fulfillment.
So when does it pay to be cautious, and when should you take a risk? The general rule of thumb is that you should be cautious when you have a lot to lose, and you should take a risk when you have a lot to gain.
Let's apply this to one of the most important decisions we all make; whether to start a new relationship with a possibly-significant other. When I was younger, I was extremely cowardly when it came to taking the first steps in starting a new relationship - asking a girl out. I read books on how to pick up girls, agonized at great length over every rejection, but then realized a simple truth - when you ain't got nothing, you got nothing to lose. This didn't make it any easier to deal with rejection, but at least it made me realize that rejection left me not much worse off than I was before the rejection occurred. Additionally, one acceptance outweighs an awful lot of rejections.
On the other side, if you are in a secure relationship, you have a lot to lose (insert celebrity cheap shot here). The calculus of caution dictates that starting a new relationship, one that places the existing relationship in jeopardy, risks losing a lot. Certainly this is in line with conventional morality, and possibly also with what psychological counseling would suggest, but I see it as simply a decision based on risk and reward. I think decisions based on a rational evaluation of risk and reward have a much greater chance of being successful than those based on feelings.