How do I stop being our group's automatic healer without wrecking the party?

I selected the archer. Three seconds later, Theo said, "Who's healing, then?" Everyone waited until I changed back.

I've played the healer for the last two campaigns. I like the class, but I chose the new game because the archer looked fun. Nobody explicitly told me I couldn't play it. They just made their preferred characters first, then pointed to the empty role as if it had my name on it.

Theo says I'm the best person at keeping everyone alive and offered to help me get the healer's gear. That is flattering. It also solves a problem I wasn't asking him to solve. I want a turn at aiming, not a better version of the same job.

We're still before the first mission, so nothing has been built around these characters yet. How do I ask the others to share responsibility for the party without making one of them feel I've forced an unwanted role on them?