
Here’s my reasoning:
Leonard is Carrie - They are the affable, most expressive, most accessible members of their groups. They are also, without question, the main protagonists of their respective shows and unofficial leaders.
Sheldon is Miranda - They are both goal-driven and sometimes focused to a fault.
Howard is Samantha - They are horn-dogs who are prone to outrageousness, both in terms of speech and behavior. They’re both super suave and flirtatious (although Howard sometimes overestimates his mojo ;)
Rajesh is Charlotte - They are the most “innocent” members of their respective groups. Charlotte because of her waspy-ness, and Raj because of his shyness around women. Plus, I think the both do a lot of physically funny things. The actors who play both characters are very much aware of their faces and body language.
Don’t ask me why! I just think of the stupidest things sometimes…