By virtue of being the only female Avenger, Black Widow has now been paired with Tony Stark (initially as a romantic foil for the Tony-Pepper union until it turned out she was spying on him the whole time), Clint Barton (as his longtime partner concerned over his well-being in Avengers), Steve Rogers (as his work wife in Winter Soldier) and Bruce Banner (first in Avengers, now big time in Age of Ultron). That leaves just Thor as the only Avenger she has no real relationship with whatsoever.
Of those pairings, only one has been fully romantic. Natasha truly only has eyes for Bruce, regardless of how audiences might have misinterpreted her earlier pairings.
In all of that time, Steve has been paired with Peggy Carter and her niece Sharon (because being frozen on ice makes for icky romances?), Tony with just about anything with two legs and skirt but, most importantly, Pepper. Clint, as we learn in Age of Ultron, has a secret wife and kids, and Bruce has Betty Ross in his rearview mirror.
Point being: Isn’t it about time Black Widow got a love interest of her own?
Solution: Give her a solo movie, already!