Ticks are parasites. The only way that they can grow to adulthood is through a blood meal. When a tick first hatches, it is in its larval stage and will immediately seek out an appropriate host. Their first target are usually small animals such as rodents or birds. Some species of tick rely upon only one host. Other species of tick will use several hosts throughout their lifetime. In this case they drop off the first host and then shed their old skin and grow into a larger sized nymph. As a nymph they might try to go after a larger host, such as a racoon, skunk, dog, or cat. This is especially true of deer ticks (blacklegged ticks). After they have gotten another blood meal, they drop off the host, shed their skin and grown into an adult. As an adult tick, they are after a much larger host. Adult deer ticks seek deer, people, even dogs. Pretty much, ticks are built to find warm blooded animals. A female adult tick will feed off its host for about 10 days. Once that is done, it will drop off your body and use your blood as the energy it needs to create thousands of eggs.
Ticks are skilled at finding their hosts. They can sense the carbon dioxide you exhale and they can sense the heat from your body. They know you are near because they can feel the vibrations of that come from you walking. Ticks use their senses to find areas where a host is most likely to pass by. Once they have found a good spot, they climb up long grasses or shrubs and sit and wait. If you are the next animal to pass by them, they will grab ahold and crawl to a spot where they can bite you and get a meal of blood.