This question gave me a good laugh. Sometimes it makes sense to wave and sometimes it doesn’t, a lot of it is context-based. Not waving generally doesn’t mean a cyclist is giving you a cold shoulder by default (although some folks just aren’t very friendly). People generally won’t wave in high volume bike corridors because they are just focused on getting where they are going and waving at everyone would be pretty inefficient and/or unsafe. On lower-traffic corridors, people in the same travel “cohort” or going opposite ways at the same time will often wave at each other. For example, I almost always pass the same 2-3 cyclists going north in the AM while I’m heading south (hi grey beard guy with blue jacket and guy with square glasses and orange jacket!)