It surely feels good to be in that situation where everybody comes up to you and say that "can you please do this work i couldn't get it done and you cant resist but to say yes .The feeling of being appreciated and called that "I just don't know what I would do without you" is surely makes you feel the importance of yourself .
A good guy syndrome is what it is called .
But the problem with this is your own important work and time for yourself is gone ,you end being more stressful then you ever will e if you would have just said no that time .

Saying yes is not a bad thing we should always up for helping people if they need us but it should not be at the cost of our own work and energy .*One more side effect of "cant say no" is people are less likely to take no from you and a result they develop the habit of always hearing yes from you and the moment you say no they are more likely to get offended then not and this can sour up the relationships between you and the requester.