People respect a “let me think about it” far more than a yes you take back later.

Agreement is a promise, and a withdrawn one costs other people their plans. They staffed around you, told somebody, stopped looking. Asking for a day feels less obliging in the moment and protects everybody, because an answer given after thinking is one you can keep. Reliability gets built out of refusals as much as agreements.

When you feel yourself about to agree just to be pleasant, ask for a day. Check it against your real week, including travel and sleep. A clear no now is kinder than cancelling on Friday night, and people remember which one you did.