Relation Ship Problems?

i havent had a girlfriend for about a year now because now i usual just flirt on wih girls and that but the problem is ive been asked out by a few girls and even if i like them i say no and i dont know why , im joinin the army in a few months so i dont know if i should be gettin into any relationships , but i dont know even tho i really liked 1 of them and she asked me out i stil sed no , what could the problem be? thanx ppl