Why do box elder bugs clump together?

Reproduction, security, solidarity, and survival are reasons why box elder bugs clump together.

Specifically, the insect in question (Boisea trivittata) adheres to the generality "Like likes like!" Getting together in groups increases the chances of mating and producing subsequent generations. It facilitates defensive maneuvers and offensive actions against predators and prey. It increases the pool of "communicated knowledge" regarding amount and quantity of predators, prey and shelter. It lets the box elder bug do what needs to be done - with lesser chances of being devoured or killed by predators and with greater chances of staying protected and warm during inclement weather - as it carries out life-sustaining activities or shuts down on living.