An end in itself
Definition 1 of 1
Pro Tip 1/3Personal Satisfaction
Use the phrase to express doing something for personal joy, not external rewards. 
She studies art as an end in itself, not for a career.
Pro Tip 2/3Misinterpretation Alert
Be careful not to confuse 'an end in itself' with being selfish; it refers to personal fulfillment. 
Volunteering is an end in itself, enriching my life deeply.
Pro Tip 3/3Subtle Motivations
The phrase can suggest deeper, perhaps philosophical motivations for actions. 
For him, exploring nature is an end in itself.