A lot of stuff goes down in the last chapters of The Selection. We see the return of Aspen, who immediately restarts his passionate relationship with America. We see a brutal and deadly rebel attack, which brings America closer to the other girls. And we see America finally accept her feelings for Maxon and approach their relationship with open arms.
In the closing scene, Aspen bursts into America's room under the assumption that their make-out sessions will continue as before. Yeah, it's not so simple. Even putting aside her growing feelings for Maxon, America has matured into a confident young woman over the past few months. She can't be expected to throw away everything she's been working toward just because Aspen comes knocking.
In fact, America says it better than we ever could:
"No, I'm not choosing him or you. I'm choosing me."
That was the truth at the core of everything. I didn't know what I wanted yet, and I couldn't let myself be swayed by what was easy or what someone else thought was right. I had to give myself time to decide what was best for me. (25.33-34)
Now, it's not like America has actually made up her mind yet—gotta sell those sequels—but this is a pretty huge moment for her, even so. For the first time in the novel, she's actively taking control of her own destiny and fighting for what she wants. It's a big step.