If burning methane is a favourable reaction, why do have to light a match to make it burn?

CH4+2O2->CO2+2H2O

Hydrogen and oxygen
react to form carbon dioxide and water under certain conditions.
 

It is important to remember that all chemical reactions
take place under specific conditions.  If the conditions are right, the reaction becomes
favorable.  For example, some reactions take place only in a certain pH range or a
certain temperature range.  We typically see the combustion of methane and oxygen when
energy is added in the form of heat.  You have probably experienced this when you light
a Bunsen burner.  Friction from flint is typically the catalyst to this
reaction. 

Hope this helps!