Your brain's reward system is a network of regions that releases dopamine in response to rewarding stimuli. Think of dopamine as your brain's 'want' signal. It doesn't create pleasure so much as it creates the motivation to seek pleasure.
When we enter into a relationship, our dopamine levels skyrocket to levels comparable to cocaine use. This much dopamine feels great, gives us energy, and actually lets us operate on less sleep, which is convenient for late-night messaging or other activities.