Is there an idiom for "People very quickly get used to good things"

Solution 1:

"take for granted"

would be appropriate. From Oxford Dictionaries:

Fail to properly appreciate (someone or something), especially as a result of overfamiliarity. ‘the comforts that people take for granted’

Solution 2:

Might be unfamiliar, depending on your audience, but hedonic adaptation (or 'the hedonic treadmill') is perfect for this. Wikipedia:

The hedonic treadmill, also known as hedonic adaptation, is the observed tendency of humans to quickly return to a relatively stable level of happiness despite major positive or negative events or life changes. According to this theory, as a person makes more money, expectations and desires rise in tandem, which results in no permanent gain in happiness.