Emotions are the whole-bodied (inner and outer man) response of a person’s values in and evaluations of given circumstances. [That’s a paraphrase from Brian Borgman.] The problem of our emotions is that they have been skewed by the Adamic fall and curse. They no longer operate righteously at all times. They are sorrowful when they should be joyful and joyful when they should be sorrowful. They are angry when they should be humble and humble when they should be angry. And so the story goes.
Jeff Forey helpfully articulates the particular way that our emotions are perverted — they are self-serving rather then God-serving and God-exalting:
“Before the Fall, emotions would have functioned in ways that supported God-honoring living. Since the Fall, they have functioned in ways that reflect sinners’ desire to be independent of God. This means that (1) although not all emotional experiences will be obviously destructive or wrong, (2) they will ultimately serve the creature and not the Creator.…In general terms, if the treasure of one’s heart is valuable only to the person and not to God, then it is unrighteous, and it will prompt speech, thoughts, behaviors, and feelings that are like ‘a polluted garment’ to him (see Isaiah 64:6). Sinful (self-serving) emotions will prompt sinful actions; not having a proper relationship with God will result in sinful reactions to life circumstances.” [Forrey, “The Biblical Understanding and Treatment of Emotions” in Christ-Centered Biblical Counseling, 399, 400.]
And to the extent that they are twisted towards self-service, our emotions cannot be trust and must be changed to Christ-like emotions (and yes, emotions can be changed).