Why is it hard to make friends?

So, why's it hard to make friends? I'm young/ish still and i think it's because everyone has grown up together and because i moved around i lack social skills and never settled. The place we did settle, i had lots of mates and was accepted. Why do i find it hard to make connections? Do other people find that? I can confidently talk to someone at a bus stop or train, but at work or in the pub, it's difficult...