Chapter 19 spoilers
Maybe I'm just stating the obvious but I feel like Leo is the reason Sho started cooking. I haven't seen anyone talking about it, maybe I'm just delusional or I misunderstood something but hear me out :')
Sho was well-off and did not have to cook for himself. He clearly says that he used to eat whatever was in his plate. He did not care about its nutritious value. Cooking for himself would have been a waste of time and energy when a housekeeper is paid to do it.
So what changed? How did he go from being disinterested about cooking to operating a food truck? It's not about making profits on his cooking skills. He is really knowledgeable about nutrition. He always comment on the quality of the food he's served during missions. He wasn't like that before. What would made him care about food?
I'd argue it is Leo.
This dialogue implies that Leo did not have a varied diet when he was younger. The nutritional value of hotdogs and yakisoba buns is pretty limited. While there is such a thing as only eating your comfort foods, Leo has been through some tough times, as hinted by his speech to Alan. Him being poor and not able to afford a lot of nutritious food doesn't sound far-fetched to me. He was visibly malnourished. Sho's comment make it clear. He could have been too poor to afford something else.
Sho could have taken pity on him, wanting to give him something nice to eat and discovering his passion for cooking. He enjoys cooking for others. He started to think about opening a food truck after MC told him he could sell his food (Episode 2 + Affinity chats), showing that he takes into account other people's opinions on his cooking. He links cooking with sharing. It would explain why he was initially not interested in cooking. *He* did not benefit from learning to cook. But *people around him* did. So he started doing it for them.
This fits well with Sho's attitude. We've seen him befriending people through food multiple times. His relationship with MC was tense until he offered her a sandwich in Episode 2. This was the first time he truly talked to her. Same thing for Subaru. They became friends thanks to the food truck.
Leo could easily have been the first person he connected with through cooking. They hanged out before but Sho giving Leo food was a way for them to be closer. Whether or not my theory is right, food plays an important part in their relationship. One of their school core memories was eating together. It was a way for them to connect.
Another fact that could support my theory is that Leo eats for free at Sho's truck. I need to check this again but if I remember well, it is said in the main story or in a campus story that he gets free food every lunch. I'm not saying that Sho never gives free food to other people. It happened with MC in a campus story and in Episode 16. But Leo seems to be the only one consistently exempt from paying. In Episode 16, MC insists to pay Sho back for the meal. It complies that she always/very often pay for the food.
Leo is a very famous Tiktoker. If he has the money to modify his dorm room, he definitely has the money to pay for Sho's food everyday. I think that it is his past food insecurity that make Sho unwilling to take his money. That or Leo hot used to the free food— Take this argument with a grain of salt until I find the source. I don't think I'm misremembering that but you never know.
When you add all of these facts up, it wouldn't surprise me if Leo was indeed the reason behind Sho's hobby. Let me know what you all think!