I'm into one of my friends
What do you think would happen if they do not feel the same way?
I won't be with them if they don't feel the same way.
You two are friends, and you don't want to destroy your friendship at a possible romance. Things can never be the same once you confess, you can never be friends again.
Why would it ruin a friendship?
It just could if things didn't work and it was uncomfortable to remain friends. Then your friendship would be ruined. So it is good to be careful when attempting to be more than friends.