Root canal or second opinion?

I went in to the dentist today for a crown that feel off a tooth and my dentist says I need another root canal on the opposite side upper Mohler. I have no pain at all in it or sensitivity to hot/cold, it does not hurt even when i push on it.. No pain at all, He told its a large cavity but im wondering why i can't just get a filling in it? this is the first time I have been to the dentist and i'm just no convinced a root canal in needed, no xray of the tooth was even done. He just looked into my mouth and said that a root canal was needed. What do you guys think my next move should be? New Dentist or just let him do it?