Quite an intriguing line. It could mean that Sherlock is not the first difficult and unsociable man John has met and become friends with. But the fact that Mary equals herself who has a sexual relationship with John and has just married him with Sherlock is indeed telling. 
This is one of many ambiguous lines in the show that may be interpreted in various ways and that make the scripts so interesting and open to discussion.

Indeed it is. 

I don't think he loved Major Sholto as much as he loves Sherlock. One of the reasons Sherlock reacts with jealousy is that John talked about Major Sholto to Mary (instead of talking about Sherlock all the time). But from John's point of view - he probably did that because he didn't want to talk about Major Sholto to Sherlock, because he knew Sherlock would overreact and he wanted to avoid that. He made a mistake unknowingly.

Or to elaborate a bit further, the first person that John felt a deep respect, loyalty and admiration for a would go to great lenghts to suit them..

QuiteExtraordinary wrote:

Zatoichi wrote:

Neither of us were the first to be John's commander?

 
That's one of the conversations in TSO3 that made me laugh at first, but in hindsight it's actually rather sad. "Your previous commander. I meant ex." - "Because previous suggests that I currently have a commander. Which I don’t." Sherlock thinks Major Sholto once had the same place in John's life as Sherlock later had. So if John says he currently doesn't have anyone who means so much to him, that must feel like an indirect rejection to Sherlock. Although John probably didn't even mean it. He took it too lightly.

"Of course you don´t." - Oh yes, this was really sad. And then Sherlock left and could not hear John saying there were two people that turned his life around. 

I think this is exactly why Sherlock is jealous of Sholto but not of Mary. Mary and John share something he never could have and was never interested in (yeah, I KNOW Johnlockers.. ), but the special place major Sholto has in John´s life is exactly the one Sherlock wants for himself.

SherlocklivesinOH wrote:

Harriet wrote:

Well, I think it is obvious that John first broke Sholto's heart and then Sherlock's. Well, probably not first.
His pattern in love issues. Like the saying "Wash me but don't make me wet!"

How would John have broken Sholto's heart? Just by "leaving him" (by being out of the army and no longer under Sholto's command)?

And since when has John broken Sherlock's heart?
 

Harriet wrote:

By making him hopes and then leaving him.

Nope. That only works if you sign up to a totally sentimental soap opera hinging on two incredibly sentimental characters. Fortunately the writers know sentimental soap opera when they see it, as do the actors, and none of them want to do that sort of show.

I'm deeply grateful for that...
 

Your point of views, sorry. No need to be rude though, Willow.

Regina Alexandra wrote:

I see it in simple terms. Mary is not the first love of John's life. Sherlock is not his first best friend.

...and John is not the first love of Mary's life. John looks so much like David.



Maybe John's just Mary's type... Which could be quite useful for Mary in case John turned out not to be Baby Watson' s father. On first sight, nobody would notice.
 

Sherlockismyfix wrote:

Regina Alexandra wrote:

I see it in simple terms. Mary is not the first love of John's life. Sherlock is not his first best friend.

 
Yes, this is how I took it as well.  John is a decent man with deep loyalty to those he admits into his life, whether it be lover or friend.  Of course, Mary and Sherlock, whom John just met in the last few years, would not be his first lover or his first friend.  They simply continue the pattern of loyalty and devotion that makes John such a wonderful character.

 
I basically agree with this (on the surface); but the sexual innuendo within the line is quite hilarious, and again, there's a certain ambiguity of interpretation that could be made if one really wants to...