Why are alkenes and alkynes more reactive?

You must first understand the fundamentals.

The most basic explanation (which you would hear in chem 101) is that fats contain more energy than carbohydrates because fats are less oxidized and therefore have higher potential energy. This is because fats are more reduced (less saturated) than alkanes (and alkenes as well), which means that there is more potential for hydrogenation (adding of hydrogen) and more potential energy to be released from such a reaction. The more bonds formed, the more energy released.

Pi bonds are much weaker than sigma bonds and are therefore easily broken. Alkanes are formed through sigma bonding. Alkynes exhibit two pi bonds, which are extremely electron rich. When you have a large concentration of electrons, it will want to balance the charge so it is extremely nucleophilic.

references from my textbook

Alkenes and alkynes are more reactive than alkanes because they have carbon-carbon double or triple bonds, which contain pi bonds that are more susceptible to attack by reagents compared to the single bonds found in alkanes.