Alex Bledsoe has written a great blog post swiping at the annoying "new love interest in every installment" trope, as well as "Women in Refrigerators" syndrome.  Definitely worth a read (as is his Eddie laCrosse series).  I already determined some time ago that if I ever write a sequel, I won't come up with a new love interest for the main character, and I'm glad I'm not the only one who feels this way.

http://alexbledsoe.com/2014/01/09/tropology-heroes-and-girlfriends/

(BTW, check out Bledsoe's "Eddie laCrosse" series.  It's very well-written.)