Why is it so hard to tell if a girl is interested in me?

I'm 29 and I've been single my whole life. I've had hookups who were only connected through mutual friends throughout time but never have I had a girlfriend.

Why? Because girls never show me signs they are interested. I don't get the smile, look away, smile, I don't get the playing with there hair, etc.. I'm a pretty average guy so for the most part I always play it safe. Am I the reason that I'm single or do women in general just don't think much of me?

Why is it so hard to tell if a girl is interested in me?
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