Hi all, I asked the professor I've been working with if he's willing to write a letter of recommendation for me (I'm applying to REU programs). He agreed and I've submitted two applications so far(one at my current university where I'd be working with him, and one at a different university). He told me he isn't willing to recommend me to any other REU programs. Can anyone explain to me why?? He already has the letter written, so it's not like it would be too time-consuming to submit the letter.