Trying too hard to be happy can make you less happy
Burkeman opens with what he calls a backwards law: the harder you strain to feel a particular emotion, the more that striving highlights its absence, creating a loop where the pursuit of happiness generates its own anxiety. Someone repeating affirmations to feel confident, for instance, is implicitly reminding themselves that they currently don't feel confident, which can deepen the very insecurity they're trying to banish.
He draws on psychological research into suppression and monitoring, where actively trying not to think about something, or trying to force a feeling, tends to make that thing more mentally present, not less. The mental effort of continuously checking whether you've achieved happiness yet becomes its own source of restlessness.
His proposed alternative isn't despair but redirection: attend to what you're doing rather than constantly auditing how you feel about it, since the audit itself is often the problem. Takeaway: chasing a feeling directly tends to chase it further away; attention placed elsewhere often lets it arrive on its own.