But what he didn't realise was his "brilliant" words of wisdom would resonate with people everywhere.
He posted a series of screenshots from an iPhone notes page, where he had jotted down his 38 points.
The list was peppered with practical tips like, "Try to be on time for things", "Take your backpack off on busy trains" and "Stop avoiding the dentist".
Declan also encouraged people to "Get a therapist, if you can."
"Even if you think you don't need one. Especially if you think you don't need one."
He warned friends with kids "need your help".
"Don't believe them when they insist they have it all under control," he wrote.
"They're exhausted. Offer to babysit."
Tip #23 on Declan's list was a major crowd-pleaser: "Dogs should be compulsory on public transport during the morning commute."
But his comprehensive list also delved into deeper waters, featuring reflections on more serious issues like mental health, his father's passing and relationships — topics that resonated with people around the world who described his take on the world as "quite beautiful".
Declan wrote he had only learned to cook in his 30s, was resigned to walking away from books and TV shows he didn't like and was OK with taking antidepressants.
"My default position in life has been: have low expectations, then you can never be disappointed," he wrote.
"Won't lie, it's not a great approach to life.
"It'll keep you safe and cynical, but that's about all."
Tip #19 on Declan's list prompted a global effort to find him a partner after he candidly admitted he had never been in a relationship that lasted longer than three months.
"I sabotage everything that even comes close to being a relationship," he wrote.
"I struggle to even go on dates. I'm working on it, I swear."
People were quick to throw their support at Declan and were soon advocating to set him up with someone as soon as possible.
Declan said he was overwhelmed by the "huge response" to his list of life lessons.
"Thank you all for being kind and thoughtful," he wrote.