It's hard to see on that tiny JPG (21kB!) but it looks like you have a generalised gingivitis with the other teeth, and this one in particular. If you aren't using floss every day, then this is when you should start.
Anatomically, the 1st premolar that you show has a groove down the mesial edge that would account for more plaque build-up here than with other teeth. But floss, floss, floss and it should improve.
Also check your brushing habit. Circular motions with a soft tooth brush is a lot better for your gum health. And make sure that you go for regular professional cleanings with your dentist or oral hygienist, at least once every 6 months.