when you approach every relationship with the thought that this couldn't possibly be 'the one' lying on the back burner, it seems inevitable that that same thought will ultimately play a part in the relationship's demise. any event that doesn't go according to plan and every trait that doesn't fit into your perfect picture becomes evidence of incompatibility. you set yourself up for failure.
disclaimer: this is a random musing, not a reflection of my personal reality.

i can see where you're coming from, rae, but i don't think it has to be that way, exactly. instead, why couldn't your back-burner thought be "hm, this could possibly be the One...just not right now"? then every trait that falls short just proves that the timing isn't right, and everything that makes the grade adds a little more credibility to the idea.
non-disclaimer: this is not a random musing, this is a reflection of my personal philosophy.