Book - Essentialism.. the disciplined pursuit of less argues that saying 'yes' to requests is based on social pressure: We say yes to most requests because we are eager to please and make a difference. However, this fills up our schedule and lives with commitments that keep us from doing what's truly important.
Keeping up boundaries and achieving more will lead to more respect in the long term, even though people will react mixed to rejections. To make this non problematic in personal relationships, we need to think and communicate in a way that separates the decisions form the relationship.
We will regret saying no for a minutes but saying yes for days, weeks, months or even years, see: The most dangerous word in one's productivity vocabulary is 'yes'
Interestingly, Carnegie recommends Get the other person to say 'yes' as soon as possible as a way of persuasion, which further highlights how the social pressure to be agreeable and consistent shapes our actions.