Thomas is not to blame for this label. He made a reasonable statement in an unreasonable, once-in-a-lifetime resurrection situation. What's fascinating is how comfortable we are in letting Thomas be so trapped.
Jesus has been their life for these last three years. They have left family and jobs -- everything -- to follow him and assist him on his mission. And now he is leaving? He is the one they believe to be the Messiah, the Son of God. How can his enemies win? They are confused, discouraged. So Jesus speaks much-needed words of comfort to these men whom he loves.
When Jesus says he is both “the door” and “the good shepherd” he is using two analogies in order to help his listeners better understand the nature of who he is and what he came to do. He’s a door because he is the way to salvation. He’s a good shepherd because he keeps, protects, and guides his sheep. He’s describing two aspects of his nature and so, we need to resist the temptation to over analyze and over-interpret this passage.