why does the stability of hydrocarbon decreases when the no of bonds between carbons increase? if there are more bonds then shouldn't the bonding be stronger ??

Dear Student
 In hydrocarbon only carbon and hydrogen are present when number of carbon carbon bond increase then number of hydrogen is decrease so multiple bond hydrocarbon are unsaturated and they are less stable than carbon carbon single bond.
 When bonds are more then carbon carbon bond distance is less this should be more stable but due to unsaturation stability decrease.  
Regards