From what I know and have experienced both love and hatred need a reason. Im going to start with hatred.
Lets say you see someone walking by you, you cant just hate the this person unless he has done something to you. Lets say the same person that walked by you now bumps into you and then instead of apologizing he accuses you of being at fault. You can hate that person because of his attitude.
Another example is when you see someone who has become extremely famous by not so morally acceptable ways, in this case you can hate the person in question for breaking what you consider morally right or you can be jealous of the fame of this person to the point that you express hatred towards this person for being so famous.
On the other hand love is an emotion much more complicated than hatred, love is perhaps one of the most bizarre emotions that a human being experiences.
The reason why you can fall in love with someone at first sight is a deeply psychological matter.
Most if not all people chose their lovers by how much they resemble their parents, for instance people look for someone that will care for them and love them to an extent that their parents did, please note that if the parents in question have not taken care of their child like they were supposed to the extend that the child in question grew to hate his own parents(which is highly unlikely) they will look for someone which is the complete opposite of their uncaring parents.
So why does love in first sight exist?
MY guess would be that upon seeing one person you relate him to someone that cared deeply for you and loved you beyond your imagination which leads you to express the feeling of love towards him/her.
All those thing about relating people and bla bla bla happen in your subconsciousness which is why you are not aware of them when they happen.