why is it so difficult to watch someone suffer?

Empathy maybe.

Because you have compassion, and empathy, and you care. Never allow yourself to lose these things.

Empathy. You can identify with them and feel their pain, in a sense.

Because you have compassion and empathy.

:( sad....heartbreaking. I don't like to see anyone struggle.

empathy

It isn't hard for many folks.

Note that sympathy and empathy are different.
Sympathy means feeling sorry for someone whereas empathy means to feel, or at least know what they are feeling.

because in your view, it's probably avoidable in some way

Fear.

Thanks