There's something inherently funny about Lambert saying that you don't always have to do something alone: Strange bites down on his lip to stop himself from laughing before he starts to explain.

"For the longest time, I did always have to do something alone. When Norrell and I worked together, we worked together wonderfully. Likewise, I could always depend on Arabella. But then I was away at war or I thought Arabella died or I became cursed and I had deal with whatever problems were thrown at me by myself."

Which is how you get great solutions like 'literally going bugfuck crazy'. Strange isn't treating this as a justification, more of an explanation.