(500) Days of Summer

The film is presented in a nonlinear narrative, jumping between various days within the 500 days of Tom and Summer's relationship. This is a linear summary of the plot.
On January 8, Tom Hansen meets Summer Finn, his boss's new assistant. Tom is trained as an architect but works as a writer at a greeting card company in Los Angeles. After discovering they have a similar taste in music, they have a conversation about the legitimacy of love, something Tom believes in but Summer does not. Following a karaoke night, Tom's friend and co-worker McKenzie drunkly reveals that Tom likes Summer, which Tom asserts is only "as friends", something Summer agrees to. Despite this, a few days later, Summer kisses Tom in private at work. During the next few months Summer and Tom grow closer, despite Summer telling him that she does not believe in true love and does not want a boyfriend.
Tom shows Summer his favorite spot in the city, which overlooks a number of buildings he likes, although the view is spoiled by parking lots. After several months of dating, both Tom's friends and Rachel push Tom to question Summer where they are in their relationship, though Summer brushes this off, saying that it shouldn't matter if they're both happy. One night, Tom gets into a fight with a man who flirts with Summer, and Tom and Summer have their first argument. Though they rekindle, Summer is unable to offer stability or to "put a label" on their relationship. On day 290, Summer breaks it off with Tom after an awkward conversation in the record store, where Summer appears disinterested in Tom. Despite Summer asserting Tom is still her best friend, he does not take the breakup well, and his friends call his younger sister, Rachel, to calm him down.
Summer quits her job at the greeting card company. Tom's boss moves him to the consolations department, as his depression is not suitable for happier events. Tom attends a blind date with a woman named Alison. The date does not go well, as he spends it complaining about Summer, though Alison tells Tom that Summer had told him she was never looking for a relationship. Months later, Tom attends co-worker Millie's wedding. Unbeknownst to Tom, Summer also attends Mille's wedding, meeting Tom on the train on the way there. After getting coffee, they go to the wedding together, where they dance together and Summer catches the bouquet. Summer invites Tom to a party at her apartment when they sit next to each other on the trip home, with Summer falling asleep on Tom's shoulder. He attends the party expecting to rekindle their relationship; instead, he barely talks to Summer and spends most of the night alone drinking, until he spots her wearing an engagement ring. Tom leaves, close to tears. He enters a deep depression, only leaving his apartment for alcohol and junk food. After a few days, he returns to work with a hangover, and after an emotional outburst, quits his job. Rachel tells Tom that she does not believe Summer was "the one", and that his depression is being worsened by the fact that he is only looking back on the positive side of their experience. Rachel tells Tom to look back for where it went wrong. He rededicates himself to architecture, something Summer had pushed him to do months ago, making a list of firms, assembling a portfolio, and attending interviews.
On day 488, Summer sees Tom at his favorite spot in the city, and they talk. Tom expresses his confusion about her actions. Summer explains that Tom was right all along that true love exists, as she found her future husband though seemingly random events, saying that this was beyond coincidence. She also explainsn that she discovered someone else she knew she had feelings for, which she had never been sure about with Tom. Summer holds Tom's hand. She says she is glad to see he is doing well. As she leaves, Tom tells her he really hopes she is happy.
Twelve days later, on Wednesday, May 23, Tom attends a job interview and meets a girl who is also applying for the same job. He finds that she shares his favorite spot and dislike for the parking lots. Just before the interview, he makes a date to have coffee with her afterwards. She tells him her name is Autumn.

Question: What sends Tom into a deeper depression after Millie's wedding?
Answer: Seeing an engagement ring on Summer's hand