Secrets require cognitive effort. You need to keep track of what you can say to whom. Who told you what and when and why. It often involves lying or omitting information, both of which require additional effort and care in your conversation. I also find that it messes up some interpersonal relationships and habits. I'm not prone to keeping secrets and withholding information to my partner, family or friends, so if friend X tells me not to tell something to friend Y it forces me to act different from what feels natural.