Sure, but the guy who makes you a taco isn't your friend, he's doing his job. You have a business relationship; you give him money and he gives you food. If he does especially well, to the point where you value his service more than the price he changes, you express your gratitude by paying more. Changetip fails because it tries to monetize social interactions, which is bizarre and inappropriate. If you can't quite grasp why it's inappropriate, consider the Changetip mindset in the real world:

You're out on a date, and you think it's going pretty well. You both get on well. Your date cracks a joke about the waiter, and you laugh appreciatively. As your chuckles subside, out comes your wallet. "That was pretty good," you say. "Here, have a dollar."

You're over at your in-laws' home for Christmas. Somehow you all find yourself discussing politics, which is always a tricky subject. Your father-in-law makes a rather strident comment you disagree with, and you're not sure how to voice your response. But then, just as you're considering how to phrase your disagreement, your brother-in-law speaks up, forcefully but respectfully explaining why your father-in-law's opinion is mistaken. The old man pauses for a second, and then sheepishly admits he hadn't quite considered that side of things. You dig into your pocket, and slide three quarters across the table. "Great reply," you say. "Go buy a soft drink on me."

You're standing on a street corner, late at night. You know you're supposed to meet your friends at "Beach Bum Bradley's Bahamanian Bar and Grill," but you're right where Google Maps says you should be, and the restaurant is nowhere to be found. A passerby in an old leather jacket says, "Hey man, you lost?" You explain the situation to him, and he replies, "oh, yeah, man, Bradley's just moved to a new place, like three blocks that way. Just take a left at Grand and 7th and you'll see it." With great fanfare, you pull ten cents out of your pocket and hand it to him.

The problem with this isn't that you've offered them a small amount of money--indeed, you've given far more money than most people get for similar "services," which is to say, nothing. The problem is that you have attempted to cast social interaction as a market transaction, which is doubly insulting. It's like offering a friend five dollars to have sex with you: first, your friend is not a prostitute, and second, your friend is absolutely not a five-dollar prostitute.