Is it a sin to devote your career to serving animals instead of humans?

Is it a sin to become a veterinarian? Unsupportive people claim that all the money for schooling, time, etc. should be devoted to humans instead.

No its not a sin. Not everyone is called to the same thing and not everyone has the skills to be the same thing. Human beings are called to be stewards of creation and caring for animals is a part of that.
The complaint about veterinarians that you cite could be used against any profession that’s not a medical doctor. Why spend money on any education or institution that’s not a doctor or a hospital? Why spend money going to the movies, a game, or a show? Why spend any money on a vacation when the money could be “devoted to humans instead?”

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