Hello Rabbi, why does tzad Shava work? Here was the Tzad shava example I was given. The school is going on a field trip. Person A is a sophomore. B is junior. C is freshman. A sees juniors going on the bus and wants to go as well. They say only juniors are going. Then A sees C is going, but he is in honor roll. So he mentions this, but they say he is on honor roll. But A mentions B isn’t on honor roll. From here A makes a tzad Shava lehavdil, C isn’t a junior but has honor roll, and B is a junior but isn’t on honor roll, A isn’t a junior and isn’t on honor roll, so he can, through this logic, goes on the trip. My question is, why doesn’t A consider the possibility of C being allowed to go exclusively because he is on honor roll? And maybe really he isn’t supposed to go on the trip, but he is allowed because of honor roll

Thank you for your question. Look at it this way. (I'll give my own example). You saw that on Friday the supermarket was packed with buyers. On other days there weren’t many buyers. Among the buyers you saw Reuven buy ice cream and shimon potatochips. There are two ways to look at it. Which makes more sense? 1. Reuven happened to come on Friday because he heard that there is a sale on the ice cream. Shimon happened to come on Friday because there was a bargain on potato chips. If this is the case, levy who doesn't eat potato chips or ice cream probably won't come. 2. There's something in common between Reuven and shimon. They both have a big meal on Shabbos since they are both Jews who honor the Shabbos with proper meals. If this is the case then levy will probably also come on Friday. It makes more sense to assume that there is one reason in common to both reuvan and shimon (option 2). If there is no good reason to assume that reuvan and shimon both came, then we will have to go with the first option that Reuvan and Shimon came for different reasons. See ketuvot 32. maharam shif , Pnei Yehoshua and harav elyashiv who discuss this regarding the tosefot שכן יש בהן צד חמור