What do you think are some of the causes of Luke's restlessness?

On page 90, the author describes Luke: "He'd always felt there was somewhere else he was supposed to be, other friends he should be hanging out with, some amazing life he was supposed to be living. And now that he was living a different life, the one he'd left behind glittered in his wake like dropped diamonds." What do you think are some of the causes of Luke's restlessness?

I think Luke's restlessness stemmed a lot from his position in the family. He was the oldest child, so he was most aware of his dad's patterns of leaving wives and children behind. He was old enough to know when it happened to him, and he was angered for his younger siblings when it happened to them (even though they were too little to be angered by it themselves). He was also the oldest male in a family that had an immature, mostly unavailable father. I get the impression that even when Adrian made attempts to be "present" in his children's lives, it was very superficial. He didn't know anything personal about them, wasn't particularly close to any, and kind of just went through the motions. This was difficult for Luke to watch, and I think he kind of shouldered a "man of the house" complex to step in where his father was falling short. We also hear hints of resentment about his schooling being paid for, and him being a middle class student at a boarding school of really wealthy peers. I think his formative years were probably confusing for him for all of these reasons, which left him with a chip on his shoulder and indifference to the world.
Of course his relationship and feelings towards Maya also were very conflicting. This was his father's new, young wife and he has all of these strong feelings, that she's appearing to reciprocate. It had to have been very hard to hide, and especially to know you cannot act on it. To have Maya's death be such a sudden and tragic event in their lives would have been hardest on him, because of the way things were left. He had no closure and felt like somebody he already couldn't have was taken. It had to have felt like a double sense of loss. Because even if he couldn't have been with her before, he could have always known she was out there and had the "what if" romanticism of their love. However, with her suddenly dying it's such an abrupt end to his story that he now has to mourn completely alone and silently.

Dlpiano summarizes the situation well. One other cause of Luke's restlessness may be his lack of a meaningful career, which could have served to ground him more. Even when he gets a full-time position, it's in Adrian's firm, when what he really needed was something outside the family constellation to expand his horizon. When he falls for Maya, he is really in an untenable position; nothing in his life is outside the world of the Wolfes. He would have been better served to make a life with Charlotte, and maybe he will, later on in the epilogue world we won't see.

In a way he was like his father - looking for the perfect woman. This was part of his feeling of inadequacy. He waste one who was sent yo further education and yet was unable to find a position for himself
: living with his father, loving his father's new wife. One wonders what his future will hold.