People are social beings. They tend to do whatever it takes to be accepted in the society they live in. This idea of acceptance leads them to do more harm to themselves than they do to others. People allow themselves to get abused and taken advantage of in fear of other people seeing them as not loveable.
This fear leads to disrespecting oneself, and therefore the only one paying the price is you.
Saying know is your right. You would accept it if someone says no to you because you believe they have the right to. Maybe you believe that they might have much important priorities, or even that they might have a valid excuse. Well same thing goes your way. You have the right to say no even if you don't have a reason.
Reasons can be very simple like because you don't want to. In the show "FRIENDS" when Pheobe in season 1 episode 1 said "I wish I could, but I don't want to" people laughed. Well the jock is on the one who assumed she shouldn't say no except if she had a "valid reason" under the judgment of the one being told no.
Saying no is a habit that takes time to be formed. Train yourself that saying no is not a privilege. Saying know might lead to few people not seeing you as lovable, but it is better them than you seeing yourself as unlovable.
Say NO, it's your right.