For Perfume: The Story of a Murderer, the question is the following :Throughout the novel, Grenouille is likened to a tick. Why do you think...

The tick is a small insect that uses its sense of smell to locate victims, and then sucks the blood from that victim.  Ticks do not spend a lot of time moving around, and they rarely interact with any other animals and they wait, silently,until they fall on a victim.  They are not particularly dynamic insects, and they exist by being parasites on larger animals.  To most people (aside from the fact that ticks can carry horrible diseases) ticks are loathsome insects; Grenouille is loathsome in the extreme, and has many of the same characteristics.  For example, when Grenouille locates his victims, he does so entirely by smell.  He also had personal smell of his own, a trait he shared with ticks (who have very little odor, because they rely on being undetectable on their host's body).

Perhaps the most important way that Grenouille is like a tick is in an emotional sense.  The point Suskind was making was that Grenouille "decided" (somehow -- but remember this is a novel with many magical and fantastical elements) that he would choose "either life or love" at his birth.  His mother had intended infanticide (by abandoning him in the street directly after she delivered him,) and if Grenouille had not cried out he would have died almost immediately, as had his four siblings before him. Setting aside the idea that newborn children "decide" anything, Grenouille cried out and was found, and subsequently saved.  The fact that he was found wrote his mother's death warrant (she was guillotined for attempted infanticide,) and meant that there was no one in the world who could love Grenouille (even if it was impossible, anyway, that his mother could love him, since she wanted him to die), and his first act in the world was to be the (unwitting) cause of a death.

Therefore, Grenouille went on to exist without anyone ever loving him (Madame Gaillard, who raised him, had been struck in the head as child, and had no emotions) -- but he lived.  He waited, like a tick, for his chance to have freedom, and, eventually, to realize his talents.  But his talents were so extreme (his superior sense of smell, and his genius for identifying and combining them) that he came to learn that the best smells could only be obtained by killing human beings -- like the parasitic tick sucking its only food,  the blood of its victims.

The analogy is not exact, but it is not meant to be.  Grenouille shared many traits with ticks, but he was person, not an insect.  He was not killing people to survive, as a tick (non-fatally, unlike Grenouille) drinks the blood of its host to survive, but merely satisfying his only pleasure -- that of scent.  But the comparison is apt enough to make sense and to reveal aspects of Grenouille's character.