Why am I so shy around big crowds, but when I'm with one friend than I'm fine?

That's simply your personality, and it's perfectly normal to feel that way. Plenty of people (if not most) are more comfortable with a close friend or two, rather than navigating the social confrontation sea of big crowds, and it sounds like you're one of them. Big crowds can be intimidating, especially if you don't know everyone, because one's attention is split between so many individuals at one time, and because there's such a pressure to converse (paradoxically, there can also be a pressure -not- to converse if you're fearful of attention). I don't blame you for being shy around big crowds-- I'm a little bit that way myself. (:

That you can only answer it yourself. You know how it makes you feel and you can decide to face the fear

To me, this is one of my "features" as an introvert. Big crowds overload me and make me anxious. Being with one person makes it feel private/intimate and really cosy! I feel like I can focus on that person better and have great talks that I only want that person to know about. It's great!

You may be feeling shy because the amount of people can overwhelm you. Being with one friend can make you feel more safe as they have your full attention.

Maybe because you're scared about what others could think of you. But when you're with your friend you're sure that he/she is there for you and he/she doesn't what to judge you or hurt your feelings.

Why you could be shy around big crowds is because to you it may feel very overwhelming or stressful, if you feel this way you may be experiencing anxiety over big crowds.

People tend to be more relaxed around family and friends because you are familiar to them. People are normally more confident and themselves around people they trust

Maybe because you haven't accepted yourself completely so you get anxious in front of other people. Think, what is it that you feel you lack in yourself? Why do you think people will judge you? Everyone lacks something or the other, no one's perfect. So everyone is at the same par. But not everyone is shy. If you love and accept yourself, it will be easier for you to open up in front of people,which in turn will make people accept you the way you are and you'll become more likeable.

Well I think a lot of people feel shy near strangers, it's a lot different when we are comfortable and with a few friends.

Being around big crowds puts yourself out there to be judged and scrutinized by others. Perhaps you feel more relaxed around your friend because the threat of judgement isn't present when you're around them.