Another realised when she saw his partner playing with his nephew during Thanksgiving.
"We were visiting my family on Thanksgiving and we were outside playing with my nephew, jumping in piles of leaves, he wrote.
"I went in to go to the bathroom and when I came out, she was still playing with him and I just watched her and my nephew having fun for a minute. I very distinctly remember thinking, 'that's it, I'm going to marry her'."
When one man found out she was secretly listening to his favourite music, he knew she was the one.
"I left my favourite CD in her car (Dream Theatre's Scenes From a Memory). She wasn't a huge fan, but put up with it whenever I listened," he wrote.
"One day I was on the phone with her while she was driving and we said good-bye but she forgot to hang up. I waited a few seconds and I heard her singing along."
Another explained there were many moments throughout his life that made him realise he should "put a ring on her".
"For me there was no big 'aha' moment. It was a long string of little moments," he wrote.
"When I could look back and just seeing how easy our relationship was. Few fights and when we fought it was always constructive. We always had the big problems and discussions figured out, finances, family and our future. We were always on the same page.
"I do remember one night just before graduating and anxious about my future, I realised that as long as I had her by my side, I would die at the end of my life happy, regardless of my success. Once I realised I had a girl that I saw a future with, I put a ring on her."
Another man realised when his partner's emotional support after a death in the family was exactly what he needed.
"The night my dad died," he started writing.
"Everybody else wanted something from me; a reaction, some words, something.
"She just followed me out into the hall of the ICU and sat next to me. She never said a word. Didn't grab all over me or smother me. She just sat next to me and held my hand silently.
"It was exactly what I needed and I remember being so glad I didn't have to explain that to yet another person; that I could just sit quietly and try to process the idea of the rest of my life without my dad and have someone there so I didn't feel so alone in that moment."
Another man explained how his partner giving him an unexpected celebration made him realise she was the one.
"I'm coming up on three years clean off of cocaine. Last year I mentioned to her that I was really proud of myself cause I was coming up on my two-year anniversary," he wrote.
"We were just starting out dating but had been best friends for a while before that. But on my two year anniversary I didn't plan to do anything except give myself a pat on the back really.
"She picked me up from work unannounced, took me to my favourite park and she had bought me a really sweet card, my favourite candies and a meal from my favourite restaurant in town. The fact that she did something for me on a day where I had zero expectations of anyone doing anything was truly amazing."
One person shared the hilarious moment their father knew he should marry their mother.
"My dad told me he knew he wanted to marry my mom when the McDonald's opened in Moscow after the USSR crumbled and she ate six Big Macs in a row," the person wrote.
