Jon and Jaclyn were friends for years before they ever even saw each other as anything more.  They always had a huge mutual respect for each other as nurses and officers, but both had their own lives and plans.  They went to officer training school together in Rhode Island where they became friends and then went on to drill together once a month where they exchanged career and life advice before Jaclyn was deployed to Kandahar.

They later overlapped each other in Afghanistan where Jaclyn trained Jon as they did the exact same job while deployed. 

As Jon was in transition home they began talking again and realized they wanted to see if just maybe they thought a bit more of each other...so they went on a date.  That was it!  From that point on they've been inseparable and it only makes sense to spend their lives together since they are already best friends and understand each other in a way no one else ever will.