Introverts will never run out of 'no's. We've been training our whole lives to find a healthy boundary between necessary socializing and running ourselves ragged. As a recovering introvert myself, that 'no' is a valuable lifeline. Like a lighthouse amidst a stormy crag, introverts will seek out their 'no's and excuses in a sea of activities to save from drowning.
So if you're an extrovert wondering why your shy friend ignores your texts or avoids ever going out with you, it's because they're powerless to say no to you. Introvert friends are a wholehearted delight when they have the social energy to canoodle, but like a delicate, exotic flower, sometimes they just need to be left well alone. We thrive in our own space and make time to recoup our small-talk-meter. Trust me, when an introvert is good and ready, suddenly their 'no's and 'not tonight's' will vaporize, and we'll reunite like no time has passed.