Question

Hydrogen and oxygen are very reactive elements but why do they form water when mixed together?

Answer 1

When hydrogen and oxygen are mixed together, they react to form water.
$2H_2+O_2\to 2H_{2}0$
Formation of water is based on chemical composition. Hydrogen with oxygen will form its oxide which is water. Water is unreactive because it is liquid whereas hydrogen and oxygen are gases.
Also, the most interesting part of chemical reactions is that the products of a reaction can have properties totally different from the reactants.