I live in Australia, where there is no tipping - not in restaurants, not at hair salons, not anywhere. Good service is expected to be included in the price and staff is expected to be adequately compensated by the employer (i.e. included in the price). Good service is not motivated by tips and yet it happens all the time. (Bad service is bad for the business, of course.)
I've lived in the US for several years and am familiar with the tipping culture. Some people are just being nice/generous, and the gesture alone, or the motivation behind it, is often good. The overall end result is questionable, and probably subject to debate. The individual gesture is expected and appreciated. Importantly, and when in Rome...
On the same "when in Rome..." theme- outside the US, Americans overtipping is anything but nice.

On the subject of gifts to the FA: this is a transaction that involves 2 parties. "I am just being nice" is only half of it. The other half is what it means for the FA. It could be well appreciated, it could instill a sense of obligation to return the favour, it can make the receiver uncomfortable, it can make other FAs uncomfortable... there is a range of possible responses. I think both sides should be considered, and it's possible that a well intended gesture might have an overall negative result. IMO the safe approach is to NOT give such gifts. I also think that a kind word can go a long way and if the service is good (doesn't have to be great), I make it a point to dispense some nice words at the end of the flight.