I don't know, I never interpreted 18's stoic indifference as her necessarily being the more outspoken in the relationship. It's more of a compensation for her lack of social skills.

Sure Krillin is a rather meek individual, but that's only in the context of him as a fighter, cause most often he's pinned against immensely stronger opponents and would prefer not to die. But when they're not fighting, Krillin takes the reigns as being the sociable one. He's the only one of them with a job, a policeman no-less, he definitely has more friends, and he's usually the one to greet others during social gatherings.

The artist confirmed this is between Cell Saga and Buu Saga, and apparently Yamcha mentioned having a girlfriend by the end of the Cell Saga. But later on, never adresses anything about her or any new girlfriend. Ergo, she dumped him.

Poor guy can't catch a break.