Because it's normal for all people unless you're a psychopath. Emotions cover a wide range and unless we're very careful color our thinking based on multiple stimuli, past experience and world view, etc, etc. Almost all people develop an idea/view/belief then go looking for evidence to corroborate that idea/view/belief, it's called Confirmation Bias and that's the normal thing for us humans to do but not always the best. One way to get around that instead of looking for confirmation proof look instead to disprove it, if you can't then it's proven.
Now don't get me wrong, Confirmation Bias is not always a bad thing, we subconsciously use it every day in multiple scenarios, it's part of our hard wired "defense of self" mechanism.
The problem is that in order to properly understand this one at least needs to know what all these mechanisms are and how they work, understand how our emotions often dictate how we see things. Say someone hands us a rotten apple we typically respond with something like: Ewwww, that's nasty. That's an emotive response. Being attracted to or repelled by are both emotive responses and I'm barely scratching the surface of this subject.