Wishing your girlfriend/ boyfriend or your spouse a very happy birthday needs to have romantic overtones. Since it is a personal letter one or two personal incidents can be mentioned.
Here is a sample romantic birthday letter:
22nd June 2010
My dearest wife Sarah,
Wishing you a very Happy Birthday! I wish I could be present today to celebrate with you. But it is not to be. I know that you will dearly miss me on this day as it is the first time that I am away on your birthday. But it could not be helped.
I still remember your birthday, 6 years back, when we had just started dating. Our trip to the beach was the most romantic day ever that we had spent till date. I think that was the best day in our life. I was making plans of celebrating your birthday at that same place, but my business trip spoilt all that. But take heart, when I return back we shall have a belated birthday party at the same beach.
Make arrangements to leave the kids with my sister, as it will be just the two of us. Here I miss you a lot. I wish I could have brought you along with me. It would have been your birthday celebration as well as our second honeymoon. Anyways there is another year’s wait.
Bye and lots of love and kisses,