It's revealed in "A Storm of Swords" that Lysa's father forced her to have an abortion after getting pregnant as a teenager from Petyr Baelish (who she had a huge crush on). She hoped her father would let her marry Petyr but Lord Hoster Tully believed Petyr too lowborn for his daughter and sent him away soon after the pregnancy. The abortion, brought on by "moon tea," was particularly traumatic, though, and Lysa nearly died from it. The need for abortion was a very embarrassing fact for Lord Hoster Tully and he was desperate to get his daughter married off before anyone found out the secret.

Jon Arryn had been trying to have an heir for years and was getting quite old. He was almost the same age as Hoster actually! Jon Arryn was looking for a healthy young bride proven to be fertile, since his other wives were unable to give him children. Lysa Arryn fit the bill and united the Tullys and Arryns in time for Robert's Rebellion, revealed in "A Game of Thrones."

It's revealed in "A Storm of Swords" that Lysa had five miscarriages and two stillborn children with Jon Arryn before Robin Arryn came along. It makes sense at this point why she’d fiercely cling to Robin, who is a sickly boy. In addition to the paranoia she's going through (detailed in the books), Jon Arryn was making plans to have their son fostered on Dragonstone by Stannis Baratheon, because he wanted to strengthen the boy up. This was the last straw for Lysa, leading her to kill Jon Arryn to prevent her son from going to Dragonstone before the start of "A Game of Thrones." Robert Baratheon offered to send Robin Arryn to Casterly Rock to be fostered with Tywin Lannister after Jon Arryn’s death, but Lysa fled King’s Landing in "A Game of Thrones" before it could happen.