Why do some compounds form double or triple bonds?

Elements form multiple bonds because they need a total of eight atoms. Anything in groups 4 through 8 can form double bonds, and anything in groups 6-8 can form triple bonds.

Some compounds form double or triple bonds so they can become more stable.

Explanation:

A Lewis structure for CO₂ that obeys the octet rule and has only single bonds would be the first formula below.

The molecule can become more stable if the single electrons move into the regions between the nuclei to form double bonds as in the second formula below.

The electrons can then hold the nuclei more tightly together and make the molecule more stable.