Why are Son Gohan and Son Goten so strong? Why is Son Gohan always so strong?

I mean Son Goku trains all day long, but Gohan studies all day long. So how did he manage to be so strong when Cell or Buu arrived? I mean, both started training when they were kids? 

Is it same with Son Goten? I mean Fusion SS3 at this young age, this is quite strong.
Not sure which episodes, but it is explained a few times that each generation of saiyans grows stronger than the previous generation. This is the reason why Frieza feared them as they will eventually get stronger than him. This means if both Goku and Gohan didn't train and were the same age, Gohan would be naturally stronger.  It's the reason Gohan tends to catch up quicker in power levels since he is more naturally gifted. 

It is also interesting to note Vegeta is from a more genetically gifted line (royalty was the strongest saiyans) so he would be naturally more powerful than Goku if all conditions were same.  Vegeta also points out to Gohan in buu saga that he has been neglecting his training so he was able to catch up to Gohan's power level when he defeated cell. Goku like you pointed out just trains like a beast even when he is dead so he tends to eventually surpass everyone.