Why is 2 years apart the usual trend?

These days I've been seriously questioning why it seems that the most popular age difference in siblings is about 2 years?  I can tell you that in my grand plan I was going to have at least 3 years difference and still ended up with barely 2 1/2 years between my children.  That baby fever hit me earlier than expected.  Is it that???  Is biology to blame for this?  The reason I'm questioning this so hard these days is because, my children are both at difficult ages at the moment and I'm going more INSANE everyday!!  They are each challenging in a different way and it's not like they take turns.  It's always both at the same time......needing, challenging, testing, risking life and limb (even if they don't see the danger).......Could it be that if we just managed a larger or smaller age difference the challenge wouldn't seem so great?  I know all children are different.......there are challenges at every age so, it wouldn't necessarily matter.......you can tell me all of that but, I've heard it before....somehow, someday's, it still feels like whether it's biology or just following a terrible trend......I chose the hardest path and the worst age difference for my children.  Why is it that I don't think it's going to get any better when they are 4 and 6? As far as the adolescent years, lets just not go there......I'm already with one foot in the door of the looney bin ;o) LOL

This is one of my favorite photos.  Such genuine love and affection

Captain Mischief himself!