Isn't it simple?

"You know I just realized." 

"Realized what?" 

"That its so simple." 

"What's so simple? " 

"We need only remember to board the bus." 

"Umm, do you need a doctor?" 

"Haha, no. Wait, let me explain." 

"Listening." 

"We're standing in a bus stop. Listening to music. Like usual, like always. Someone comes by, and starts a conversation and you take off one ear of the headphones." 

"Okay?" 

"Its usually boring. So we go back to listening to music after a while. But sometimes, the conversation gets really interesting. You are enjoying it." 

"What does this have to do with the bus?" 

"The point is, after a while, that person boards the bus and leaves." 

"So?" 

"So we must board a bus as well. Not one that the person before boarded, but the one we were originally supposed to." 

"What if I forget that during the course of conversation? You said its so interesting and all that." 

"Yes precisely. That is why we must always remember to board the bus." 

The bus came. They boarded it.