I think we can attribute Frank's personality to the following points:
1. Frank is rather introverted. After his family moved away, this left him quite uprooted and insecure. I think the friendship with Jess, who was more relaxed and confident than Frank, in terms of how her relationship with Shannon and her clique was a necessity, was something they both needed. Jess because he was someone new and not prejudiced against her while Jess was a good friend to him. In fact, in a non-romantic way, I see a certain similarity to the friendship Frank has with Vanessa. But well, maybe Vanessa and Jess just have similar personalities.
2. everything negative about Jess: first that Jess was disowned by her family and to see a friend who was always very strong break down because of it and then years later her much too early death, which makes him an overprotective father. And then that the guilty party gets away scot-free.
3. all about Shannon. Both in the distant and recent past. I don't even blame Frank for marrying her, Abby needed a mother figure and for a while Shannon actually did that relatively well. And let's face it. Frank explaining the changes in her body to a pubescent girl would have been "awkward".
There is no question that Becca and Lilly also needed a father figure. It was also helpful for Abby to have girls around her who were around her age, because I think that when it was just Frank and Abby, she would have had very few friends.
In short, Frank was an introvert who, through a series of circumstances, was in a situation that few would have coped with and made the best of it that he could. Did he always make the best choice? Definitely not, but that's also part of growing up.
