This is because, according to most modern perspectives, pleasure is an important component of happiness.
All pleasures, from the most fundamental (food, sexual pleasure) right through to higher-order pleasures (e.g.
monetary, medical, and altruistic pleasures) seem to involve the same brain systems (Kringelbach & Berridge, 2010).
Perhaps, though, as argued by Ed Diener, happiness is subjective.
According to Ed Diener, people are happy if they think they are, and each person is the best judge of whether they are, in fact, happy or not (Norrish & Vella-Brodrick, 2008).