Only Jonathan Frakes could have been second in command

Not anyone could have been second in command aboard the Enterprise to Captain Jean-Luc Picard, but Jonathan Frakes did. 

Captain Picard was nothing like Captain Kirk on Star Trek: The Next Generation. He wasn’t the type to joke around or relax the authority structure. That’s why The Next Generation needed a second in command who could essentially be Captain Kirk at a lower rank.

In Center Seat: 55 years of Star Trek [via Movieweb], Ronald D. Moore explained that Jonathan Frakes’ role was a difficult one. He didn’t have Spock’s Vulcan heritage to fall back on, and he certainly wasn’t a close confidante to Picard…at least not right away. So he had to create his own character in Commander William T. Riker, and do it in such a believable way that people would understand why he’d turned down being a captain to stay aboard the Enterprise.

"“It’s a difficult role to be second in command on that series, if you’re not the Vulcan science officer. Because the Vulcan science officer has a whole rich cultural thing as a character that you can get into, that’s providing a yin and a yang to McCoy and a confidant and friend to Kirk. So that functions differently. Now, it’s a human person, who’s supposedly just as ambitious, just as capable and dynamic a hero in his own. Right. Okay. Why is he staying to be number two? It’s a tricky role to pull off, but Jonathan pulls it off because Jonathan is so innately charming and fun, and you really, really liked him. You like Riker and you’re intrigued with his backstory with Troi, and again, the force of the actor brings the character home.”"

It’s hard to imagine anyone by Jonathan Frakes being Number One.

And one of the best things about Commander Riker was his ability to adapt to situations quickly. That part of him only grew when he became a captain and recently joined Admiral Picard aboard the Titan for what might be their last adventure. (One can only hope not.)

What Moore said is true. Because of Frakes’ own charm and fun nature, it was easy to like him as second in command, and no one questioned why he didn’t want a ship of his own for seven years. Unfortunately, we didn’t get to see him captain his own ship much, and that’s something I hope we’ll be able to see, perhaps in Star Trek: Legacy.