Experiments in animals idetified areas of the brain thet seemd to induce a pleasurable state.
If an animal was trained to press a lever to recieve brain stimulation in these areas, they would choose to bar press over the food, water, or sex.

Such animals will self-stimulate till exhaustion! (this experiment has relevance to human addiction)

What is rewarding brain stimulation?

Pleasure Systems in the Brain
(Michael A. Bozarth)

… Olds and Milner (1954) first identified brain sites where direct electrical stimulation is reinforcing. Laboratory animals will lever press at high rates (> 6,000 times per hour) to obtain brief stimulation pulses to certain brain regions. The reinforcement from direct electrical activation of this reward substrate is more potent than other rewards, such as food or water. The potency of this electrical stimulation is most dramatically illustrated in a classic experiment where the subjects suffered self-imposed starvation when forced to make a choice between obtaining food and water or electrical brain stimulation (Routtenberg & Lindy, 1965). A second distinguishing feature of reward from electrical brain stimulation is the lack of satiation; animals generally respond continuously, taking only brief breaks from lever pressing to obtain the electrical stimulation. These two features (i.e., super-potent reward and lack of satiation) are important characteristics of direct activation of brain reward mechanisms… (full article)

TTC Understanding the Brain – Lecture 24: The Reward System Anatomy (from guidebook) here

TTC Understanding the Brain – Lecture 25: The Reward System – Drugs (from guidebook)
here and here

More about TTC Understanding the Brain (36 lectures video course)