Why do guys only seem to like outgoing girls?

I'm 22 years old, and in college. I have had boyfriends in the past, so I have reason to believe my looks are at least decent. Without tooting my own horn, I do think I have plenty to offer a guy, yet they always seem to prefer really outgoing, flirty girls. I'd hate to change myself to please guys, since whenever I try to be bubbly it always looks really forced and unnatural. I am generally friendly, and can hold a conversation well, it's just that initial step of getting them to talk to me that's difficult! I feel like I'm missing out on many romantic opportunities because I'm not "out there" enough. What is it about more reserved girls that puts guys off?
Why do guys only seem to like outgoing girls?
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