It's not the memories of fighting in the Middle East that cause Ivanhoe most of his heartache; it's his struggles with his dad. The fact that he chooses to fight in the tournament at Ashby under the name "the Disinherited Knight" is proof of how important this is to him.
In fact, for a novel that claims to be about big historical issues, a surprising amount of the plot revolves around personal family problems. Rowena wants to marry Ivanhoe, but Cedric (her guardian) engages her to Athelstane. Cedric blames Ulrica for living on after the deaths of her father and seven brothers, even if it means she can help out the Saxons at Torquilstone. And Richard I (the king of England, for Pete's sake!) has to step in to make things right between Ivanhoe and Cedric. This focus on individual relationships between characters makes the larger historical events operating in the background feel more personal and significant.
The Knights Templar and the outlaws of the forest both show more dedication to one another than Cedric shows to Ivanhoe or Prince John to King Richard. By emphasizing the value of personal bonds over blood relationships, Ivanhoe argues that family ties are ultimately less meaningful than attachments of choice.
Rather than being a support network, family in Ivanhoe becomes another obstacle for the heroes of the novel to overcome.