This is a very complex and controversial question which takes into account a lot of different factors. For instance, does a person's intelligence level have to do with the genes that they inherited from their parents or does it have to do with the environment that they live in? Both of these factors can significantly influence the level of a persons intelligence.

The definition of intelligent or smart is very broad. The definition for smart in the online Merriam-Webster dictionary is mentally alert whereas the definition for intelligence is the ability to learn or understand or to deal with new or trying situations, or the ability to apply knowledge to manipulate one's environment or to think abstractly as measured by objective criteria (as tests).

Basically, intelligence can be measured by tests and the ability to reason. So why are some people smarter than others? I think some people are more smart or mentally alert because they live a balanced and healthy lifestyle with regular exercise, eating the right foods, and getting enough sleep. Also, emotional factors, like a persons level of happiness, play a role in how smart they are.

As for intelligence, I believe some people are more intelligent than others due to experience in life (the older you get) and the more the person studies and progresses in education. You heard the saying practice makes perfect. Well, that's how it goes with intelligence too. The more you practice thinking and reasoning, the better you will be at it, and the more intelligent you will be.

The topic of individual differences is quite interesting in the field of cognitive science and human factors. Keep in mind that intelligence can be broken down into various different types. For example, there are people with high verbal abilities and others that posses high spatial and visualization abilities.

Your question is rather controversial and relates to the age old nature versus nurture debate. The controversy is whether people are born with innate abilities or whether we develop abilities as a result of the nurture we receive.

To answer your question, some people receive many resources when growing up that leads them to perform better at certain things. Some parents read to their children at a very young age, while others encourage their children to solve jigsaw puzzles or play tan grams. The latter set of activities is much more spatial than analytical. So as a result of the training you receive, and to some extent your family history, people have different types of intelligent and might SEEM smarter when it comes to certain problems.