After Darwin's long trip to the Galapagos and back, and his constant work, he got sick from the pressure. He had a heart palpitation and his doctor ordered him to stop working and spend a couple weeks in the country relaxing and resting.
He obeyed and went to spend some time with his mother's family. It was there that he spent some time with his cousin, Emma Wedgewood, who was taking care for his invalid aunt. Although there aren't many details of their interaction, he must have taken a liking to her.
Once he recuperated from this disease spell, he jotted down a list of pros and cons for marrying his cousin. Some of the pros were: " constant companion and friend in old age...better than a dog anyhow." The cons? "less money for books" and "terrible loss of time."
He eventually proposed and married her in 1839. A third cousin officiated their wedding. The Darwins went on to raise 10 kids.