By jumping between the peak of their affection and the misery of their fallout, the film forces the audience to look for the "red flags" Tom missed.
The film’s non-linear timeline—jumping between day 1 and day 450—mirrors the way memory actually functions. After a breakup, the mind does not recall events chronologically; it splices together the highest highs (day 154: the IKEA date) and the lowest lows (day 282: the bar fight). This structure serves two purposes:
The film also touches on the theme of personal growth, as Tom navigates his career and relationships, learning valuable lessons about himself and what he wants out of life. This theme is echoed in Summer's character, who is on her own journey of self-discovery, trying to figure out what she wants and needs from life.