Is dyeing your hair THAT bad?

Well I have EXTREMELY thick, longish hair, and all my life people have told me the worst thing you could do to your hair is dye it. They say if you do it will make your hair thin and break it off, which is part of why I never thought once about dying it in the first place. Is this true?? Because I'm really getting tired of the same look and I kind of want to dye my black hair a bright red (professionally), but I'm really scared. I get a lot of complements on how healthy, thick, and long my hair is and I dont want to ruin it just because I wanted to try something new. I know other people dye their hair or whatever, but my hair grows reaally slow and I dont want to take a huge risk. Any advice? As much information please