There must be some relationship between "passion", adrenalin and endorphin. What could this be?

Short answer - Tango!

Long answer:

Epinephrine (adrenaline) is a "fight or flight" hormone which is released from the adrenal glands whenever danger threatens. When secreted it floods out the endocrine gland into the bloodstream to instantly prepare the body for action in emergency situations. The hormone boosts the supply of oxygen and energy-giving glucose to the muscles making the individual more mentally alert and physically strong; only vital bodily processes occur.

Endorphins (or more correctly Endomorphines) are endogenous opioid biochemical compounds. They are peptides produced by the pituitary gland and the hypothalamus in vertebrates, and they resemble the opiates in their abilities to produce analgesia and a sense of well-being. In other words, they might work as "natural pain killers." Using drugs may increase the effects of the endorphins. The term "endorphin" implies a pharmacological activity (analogous to the activity of the corticosteroid category of biochemicals) as opposed to a specific chemical formulation. The term endorphin rush has been adopted in popular speech to refer to feelings of exhilaration brought on by pain or danger, supposedly due to the influence of endorphins, although this term does not occur in the medical literature.

passion: emotion, a strong liking or desire for or devotion to some activity, object, or concept

It looks like our passion is giving us an endorphine rush, and inturn is created by endorphine rush combined with adrenaline rush. :-)