When he first met Sherlock Holmes, Watson was desperate to find decent quarters at a reasonable price. Though Mr. Holmes seemed a bit eccentric (beating a dead body to look for bruises) and had several rather odd habits, it was worth a try to go halves on a place with him.

When he first met Dr. John Watson, Holmes perceived that he was an army doctor that had recently been wounded in the second Afghan War. Likely to stay home most of the time due to his strapped purse and injury, Watson would be underfoot. Although that really didn't matter much, since he won't make it through the first six months.