Symbiotes of course bond to a host, but they have choices, you know.
First and Foremost of the traits that attract symbiotes would be emotion. Since emotion is caused from the excretions of certain glands in the body, one could say those who are highly upset are prone to releasing adrenaline. Often times to those certain people are easier to manipulate to the symbiote's will, therefore more vulnerable.
Then again, symbiotes don't go all willy nilly bonding to random humans most of the time. I mean, they're pretty smart. Desperation or preference can differ in each situation, like humans do. Take for example Venom. The symbiote bonds to Brocks wife, Ann, then got bought off, and then bonded to scorpion. For the most part there pretty related. For Ann, the situation was to save her from death. For the second time (I don't know his name...) he was sold off, then abandoning its host. It soon after finds scorpion, probably attracted by the matter of strength and relation to Spider Man.
Ok...Why humans?
Symbiotes would probably go to the dominant race of any planet, as that would benefit in the means in survival. In this case, humans. They are known to bond to animals as well.
So, what happens next?
After the bonding stage, the symbiote and the host are linked together mentally as I may have talked about before. This explains the speech in plural (usually) such as "we" in comics. Opinions on how it works differs. They could probably hide some of its matter inside the brain, if not on it, and enter the brains actions and patterns and control it from there. However, this conflicts with whenever some ones symbiote gets ripped off (whistle) the link is severed. Perhaps the matter in the head would die off after separated from the main mass?
Another take is the a symbiote has some degree of telepathy or psychic powers (which could be explained in a previous submission). This can suggest that symbiotes do not necessarily stay in the head, but many other places while communicating would be still possible. It still might need contact with a host to do anything about it though.
