My dad said he stopped being a professor because of that.

He liked the research, and he even liked teaching, but he absolutely hated having to constantly try and find grant money. He said he ended up seeing everyone as "potential funders" and less like "people" because his job kind of depended on it, and it ended up burning him out. He lasted four years and went into engineering.

I don't know that "motivation" is the right word for it, because I don't think professors like having to find grant money all the time. I think most people who get PhDs and try to go to academia do it for a genuine love for the subject, and they find the grant-searching to be a necessary evil part of the job; it's more "survival" than regular motivation, though I am admittedly splitting hairs here.