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

That is normal. You probably have not done much practice with public speaking. When you are with a big crowd, you feel somehow you need to present yourself, when instead it is often better to just be yourself.

Being with a friend or someone you know, you will feel more comfortable in situations that would normally cause anxiety

You already know your friend really well and you can trust him/her. In big crowds are just strangers and you may feel insecure.

You may be suffering from social anxiety and there are many ways to cope with this! I too suffer from this.

You may be shy because you are concerned about the possibility that someone may judge you. With your friend you do not have that concern.

I feel you are comfortable around your friend because you got to know them personally, but crowds can be filled with strangers, or people you probably don't know very well. It can be overwhelming!

Maybe because you're socially anxious around others. Lots of people are like that and have social anxiety.

It just indicates that your confidence level is lil bit low and you need to work on it. You must be thinking in your head that they must be talking about you or the way you walk and they must be making fun of you .. Etc etc but you need to boost up confidence level by trying to speak in front of mirror everyday for atleast 15 minutes which will help you to gain confidence and it would be helpful in other places too! I know this world is full of monsters behind friendly places but.. You need to face them alone and if anyone will ever try to harm you.. You just kick them hard and show them you middle finger and proceed walking!

Often when we are in large groups, the environment can be overwhelming. Social anxieties are amplified within larger groups, especially of those who we don't know very well. If you find things easier around good friends, perhaps go to places together, to try and help ease into the group.

Social anxiety. I have the same problem. When you are around many people you feel like all of their eyes are on you but when you are around one person you might feel like it is less judgement.