The relationship was completely unexpected and random. Jane was infuriated at their relationship, because Thora's son and Dave's daughter (herself) are already married.
Grandpa Dave proposed to Thora after on Christmas Day 2014, only speaking to her twice, and he even forgot her name. Thora was tempted but emotionally declined, mentioning her late husband. They shared one last kiss and she ran away crying (Christmas with the Reads, Part 3).
After Thora experienced a heart attack, she was rushed to the hospital and given minutes to live. Thora requested that Dave be by her side as she died. Dave arrived at the hospital. The nurse suggested that Thora could survive if she was given a heart transplant, but they had no available donor. Dave proceeded to rip open his own chest, and pull out his heart. He told Thora "You gave me a purpose" before dying. Thora was heartbroken. The nurse then added that Thora actually did not need a heart transplant, and she recovered without Dave's heart.