Don't be put off my the neuroanatomical terms mentioned above. I'm not actually concerned with "exactly" where feelings originate within the brain but more with the relationship between our attempts to come up with rational arguments or thoughts to explain our feelings.
There is no question that we are drawn to certain individuals and repulsed by others. The factors that go into these reactions are often not immediately obvious to us.
Certainly, there is a level of physical attractiveness or lack there of which draws us to or away from someone. We also speak about "personality" and multiple factors are at play here as well.
Sometimes someone will remind us of an individual you either like or dislike. This may immediately impact on how we regard them.
Then there are individuals whose voice, mannerisms, degree of animation or demeanor appeals to us. Perhaps there are smells, even pheromones as well that appeal to us on a subliminal basis.
Sense of humor, aggressive tendencies, the attention that they pay to our thoughts and comments all play into this emotional reaction–the world of feelings–that combine to our opinion of another human being.
If someone we "like" touches us, this often re-enforces our positive feelings towards them. While we might be repelled by a similar touch from someone who "turns us off".
What occurs after wards is an attempt to explain why we like or dislike someone. We begin to create pros and cons, offer logical arguments for or against specific traits of that individual.
But in many ways, these attempts to analyze with our frontal cortex are secondary to feelings which are pre-verbal. Our emotional reactions are often difficult to put into words but that doesn't stop us from attempting to do so.
So next time you meet someone for the first time or are with a group of others, try to examine what it is that you find appealing or not about them. You may take notice that it is harder to put your reasons into words than you might think.
In any case, your feelings often precede your thoughts.