IMO, the want arises from biological needs.
We love? Because we want to satisfy our sexual urges and fulfil our natural call to propagation.
We hate? Because it threatens our selves. We build a mental barrier to prevent the hurt.
We think? Because we are insecure. We want to rationalize things, because each one of us is alone in his/her own mind, and by thinking, maybe we can console our natural innate fears.
Does it require a certain brain capacity or is it related to a "soul" of some sort.
Well, having a brain is good.

Maybe you need just enough to make the vital neuron pathways, build up the different parts of a brain that responds to external stimuli (pain, etc), and you're on your way! But having a bigger brain doesn't mean you'll have more ideas or thoughts.
For example, a whale brain weighs 20 pounds. A human brain's a bit under three pounds.
"One theory looks at the brain mass/body mass ratio. The brain does more than think; it is responsible for the proper functioning of the entire body. The brain controls breathing, body temperature, heart beat, walking, blinking, all those aspects ofstaying alive that don't need conscious control. In general, the housekeeping of the body. The brain/body ratio theory basically says that the lower the ratio of brain mass to body mass, the more of the brain that must be devoted to housekeeping, although there is a certain minimum brain mass required for even simple housekeeping (for example, the African pygmy shrew has a brain/body ratio approximately equal to human beings (1:.02); few would ascribe to it equal intelligence). It would seem that any brain left over after housekeeping chores have been taken care of could be devoted to other things. Like thinking.
"Of all the animals on Earth at present, the animal with the greatest brain/body mass ratio is the human being. The average human (150 pound body, 3 pound brain) has a ratio of approximately 1:.02 (for each pound of body mass there is .02 pounds of brain mass). The average dolphin has a ratio of about 1:.00825 (the blue whale mentioned above has a ratio of about 1:.000165). The two highest ratios in the animal kingdom are the human and the dolphin (by comparison, the wolf, a very intelligent canine, has a ratio of about 1:.00156).
"Let us assume that the wolf requires its entire brain for housekeeping (a totally unfounded and, to me, absurd assumption). If this is the case, then the human brain has left over, after housekeeping, nearly 13 times thebrain mass the wolf has that can be devoted to thinking.
"Let's try an animal further down on the scale in terms of both brain/body ratio and perception of ability to think. Making the same comparison as human and wolf between a human and an alligator (ratio approximately 1:.000072), an animal not usually considered extremely bright, then the human has 278 times the brain mass that can be devoted to thinking. With that much capacity left over after housekeeping the human brain can devote itself to other pursuits. Those other pursuits are called thinking.
"To carry out the housekeeping functions of the brain, it must have input from the world outside the body it is keeping. It must know about cold or heat, it must know about light or darkness, it must know about hunger, thirst, or satiety. The brain must adjust the body to fit the conditions. For example, if the air temperature drops, the body converts more fat to energy to maintain body temperature. If the body is exerting itself, it needs more oxygen to keep the muscles functioning; the lungs automatically begin working faster and inhaling deeper to increase the amount of oxygen and the heart speeds up to deliver more fresh blood and thus more oxygen to the muscles. To stand on two legs requires balance which requires constant small adjustments of muscle tension and body position. All of the above is accomplished without conscious control. You do not have to think about speeding up your heart or adjusting each individual muscle to avoid falling over. These are housekeeping functions and you can ignore them. However, as shown above, housekeeping is not the only thing the human brain is capable of or does.
"To conceive, imagine, ideate, to think, also requires input from the world around the brain. Without that input, there is nothing about which to think."
http://www.wsu.edu:8080/~taflinge/mindwork/mawint1.html