Why is meditation pleasurable?
When I took the course Buddhism and Modern Psychology, the lecture delved into the neurobiology behind meditation. Material was presented that breaks up the conscious mind into 8 centers competing to be at the forefront of awareness. When meditating, the Default Mode Network(DMN) in the mind shows less activity in fMRI scans in studies led by Judson Brewer who himself used meditation as a coping mechanism during medical school.

The DMN is the center of the mind thought to be responsible for daydreaming. The exact mechanism between meditation and pleasure in the mind is not known, but I speculate that a sort of rhythm develops between these competing centers and other parts of the brain that leads to decreased levels of cortisol and increase in pleasure neurotransmitters such as dopamine.

A point to note: Suzuki Roshi mentions in Zen Mind, Beginner's Mind, that the purpose of meditation is not too feel good or become a better person rather the purpose of meditation is to meditate.

https://en.wikipedia.org/wiki/Default_mode_network

https://www.coursera.org/course/psychbuddhism

http://www.yalescientific.org/2012/05/the-healing-art-of-meditation/