Receiving a rejection of a request to write a medical school letter of recommendation is disappointing, but consider it a silver lining. It is far better to receive a “no” and find someone truly passionate about writing you a letter than it is to receive a letter that only includes half-hearted support for you.
If the person you ask declines to write a letter, ask someone else. That is why it is a good idea to create a list of programs you are applying to and people whom you can ask to create letters, along with backup options.