I tip if I get good service in a restaurant, or it's a restaurant I go to regularly, I'll do the keep the change bit.
BUT, there is no need to tip in Australia, our customer service industry is farily well paid, and have fairly good conditions.
If I don't get good service I do not tip.

A taxi, might get a tip if he's quick, pleasant and doesn't go the long way. I have been close to underpaying a taxi on occasion when a trip cost a lot more than it should have, because he took an odd route.

In America, as mentioned by others, their customer service industry is paid a very basic wage. the system is intended to promote good service, if you don't get good service you still don't have to tip.
The idea is it gives staff incentive to give good service so that they then get a tip, if they are always very good, they can make a tidy living off tips. so if I do get good service in the states I do give a tip. 10 - 15 % as is recommended.

in Australia my tip amount is often the loose change of the purchase, in the case of a taxi, I'll throw a dollar or 3 into the deal if I decide to give a tip. In the case of a Pizza delivery, if it was fast and courteous, they get a couple dollars change, but like also mentioned, if they don't even lok like getting chnge out, they aren't getting a tip.

it's a complex place in my head