Full Idea
In feeling towards things the imagination tends to 'run away with you', which is partly why the emotions are 'passions'; your thoughts and feelings are not always as much under your control as you would want them to be.
Gist of Idea
We call emotions 'passions' because they are not as controlled as we would like
Source
Peter Goldie (The Emotions [2000], 3 'Towards')
A Reaction
This may have the chronology wrong. 'Passion' doesn't mean uncontrolled. I take it that 'passion' was an older word for 'emotion', and became attached to the older view of emotions as dangerous and corrupting.
Book Reference
Goldie,Peter: 'The Emotions' [OUP 2002], p.58