Because that insinuates that only by having a significant can you be whole. To me, there isn't some person out there who will complete you. You are not a piece of a puzzle. You're already whole. Right now, reading this, you, your soul, your heart, and your mind are all completely, 100% whole.
I'm going to use myself as an example to get across what I'm preaching, here. Right now, as I type this, this is what my 'heart' looks like:
I have no desire at the moment to have a boyfriend. I just don't have that hole to fill at the moment. I have my books, my friends, my art, all these things that fulfill me. Later, as I grow and become even more of my own person, this will happen (I call it the Boil Phenomenon):
I will still remain the same person, whole, but all of a sudden, something will happen. Maybe someday I will be reading a romance novel, and decide that something is kind of missing. I won't lose myself, but my heart will suddenly grow and I'll have this space that will be filled by a special kind of companion (you know what I mean *wink*). Some day, I'll meet a guy, and we will talk and I'll think, "He's just the thing!" and he will fill the little growth. If something happens and we don't work out, I won't become half of a person or anything dramatic as that; i'll just go back to being 100% instead of a 110%. It will feel as though I'm not whole, but I won't be.
Later, I will see another boy, and ZOOP, the Boil Phenomenon will happen again. If this time, we fall deliriously in love enough for me to happily become a part of the ancient patriarchal system known as marriage and become Mrs. Him, then my heart will look a little like this:
That thing which began as a boil will slowly grow over my heart and then permanently meld, over time, so that I will forever be 110%.
A similar thing happens to men, I'm sure, but I won't get into that because I am not a man, so I can't technically fact check. However, my papa (the resident man for questions like these) says it's the same thing, only the Manly Man Heart looks like this:
For some people, the boil will never grow. That's okay. Sometimes, I look around me, and I see all these girls being in relationships and think "huh", but then I realize that, for some of them, they have the idea of being a part of a puzzle, and once you find that SPECIAL GUY, you will complete each other. It's a nice idea, I'll admit, and a little more romantic that a boil, but much less practical. It means that every moment you are single, you are only 50% whole, which isn't true.
You aren't looking to become 100%. You already are. You're just waiting for the boil to tell you that it's time to be 110%.
And remember, you cannot force the boil. DO NOT FORCE THE BOIL. THE BOIL COMES WHEN IT WANTS.