Friends and The Big Bang Theory are sitcoms that have a lot in common. While they come from different eras and focus on very different groups, they are both shows that centre around people. People struggling with love, people enjoying each other's company, and people trying to make a life for themselves.
Naturally, a show focussing on the everyday lives of people needs to have relatable, funny, and, most of all, likeable leading characters. That's part of the reason these shows thrived, as they hit the nail on the head with their casts. Both are full of loveable characters, and two of the best are Ross Geller and Leonard Hofstadter.
These two are very similar in many ways. They are both learned men with P.H.Ds and plenty of academic accomplishments - one in palaeontology and the other in experimental physics. However, despite their accolades, they're also awkward, goofy and prone to embarrassment, leading them to struggle with dating and plenty of other issues.
Thanks to how closely related they are, you'll probably have trouble deciding which of them said what. Thus, let's see if you can get a doctorate in Friends/Big Bang Theory fandom.