Ted has a few serious relationships before meeting Tracy (the titular character) in How I Met Your Mother - and other than Robin, the biggest was probably Stella. Although she broke his heart when she left him at the altar, for a while there, it really did seem like he had found his bride - by getting an embarrassing tattoo removed, of all things!
However, there were quite a few signs from the start that this was never going to be the relationship of Ted's dreams - and not just because it happened in season 3, and there was no way the show was ready to wrap up yet! From interventions to lifestyles that just don't line up, Stella was never going to pass the front porch test.
She Told Him Upfront She Had No Time
Clearly the biggest warning sign was that she repeatedly turned him down from the start. Initially, it was for a legitimate reason: legally, she could not date a patient. However, when his time as her patient was done, he asked her out again, and she still turned him down. She was a single mom, she was incredibly busy, she simply had no time... all things that Ted pushed past, to take her on a two-minute date, woo her, and wear her down with sheer persistence. It was played as cute at the time, but really, if Stella and Ted were meant to be, she would have made a point of finding time, not repeatedly telling him she didn't have any.
The New Jersey Problem
Stella lived in New Jersey with her daughter, and Ted lived in Manhattan. Anyone who knows New York knows that was always going to be a problem. Not only did it create huge issues for the two seeing each other, it speaks to the different kind of lives they wanted to lead. Ted may have eventually wanted a house in the suburbs, but when they met, he was still loving city life. Add in the fact that she refused to move to the city for him, but moved there for Tony, it was clear that this was too big a divide to really cross.
Ted Would Struggle With The Step-Dad Dynamic
Ted makes it painfully obvious from the start of the show that he wants to find a woman, fall in love, get married, and have kids. He wasn't this so much that he becomes obsessed with the idea of it, often making terrible decisions as a result. However, Stella doesn't really fit with this: she already has a house and a daughter, and this is a relationship that involves Ted joining her established life and family, not the two of them building a new life together from the ground up. For a lot of people, this would be perfectly fine, but Ted was always going to struggle with it.
Stella Didn't Fit With Ted's Friends
Especially with a friend group as close as the HIMYM gang, it's incredibly important that a romantic partner fit in - in the show, Lily even calls it the 'front porch test'. And Stella simply didn't fit in.
It was clear from the start, whether in how bored the gang was when they went to hang out in Stella's basement on a weekend, or in how Stella never had time to hang out with the gang at MacLaren's. They were even going to stage an intervention for Ted over Stella, which shows just how much they didn't think it would work.
Stella Was Reluctant From The Start...
Stella's obvious reluctance wasn't just in the fact that she repeatedly turned Ted down before agreeing to a date, but in how slowly she wanted things to move. She didn't want him to stay over, or to meet her daughter - and while holding back for a while makes some sense, especially when kids are involved, this was clearly more than that. She just wasn't all in with Ted.
... And Ted Was Moving Way Too Fast
While Stella wanted to take things slow, Ted was already convinced he had found 'the one' and was moving at high speed. He proposed at a point where he still barely knew her! Moving fast might not always be a problem, but this felt entirely like it was driven by Ted's desperation to be with someone (anyone), not like it was because Ted was truly in love with Stella.
They Have Totally Different Interests
At one point, it comes to light that Stella has never seen Star Wars, and when she does, she hates it. She pretends that she likes it, for Ted, and this is played up as a sweet moment - but in essence, it was still her lying to her partner because they just didn't have the same passions.
Star Wars isn't just a movie for Ted, it's very symbolic of who he is as a person, and there's no evidence that Ted and Stella have anything in common beyond attraction and a mutual desire to be in a relationship.
Stella Always Looked Down On Ted
This is a little less obvious at first, but it does become extremely clear in the end - Stella never really seemed to view Ted as an equal. Maybe it's because they met when Ted came in to have a butterfly 'tramp stamp' removed, or that he was struggling in his career while she was a successful doctor and deputy mayor of her area, but she always seemed to pity him a little bit. In the end, when her perspective shone through with 'The Wedding Bride', it became even clearer: Stella always looked down on him.
She Wasn't Over Tony...
This is definitely something that is more obvious in hindsight, but it's still worth mentioning. At the end of the day, Stella had been in love with Tony since high school, and they were co-parents to their daughter. Stella's unresolved feelings may only have become clear in the lead-up to the wedding, but they were always there. And when the right person showed up, she was suddenly willing to find time, take big leaps without waiting, and move to the city... all the things she wouldn't do for Ted.
...And He Wasn't Over Robin
Stella wasn't the only one with unresolved feelings. Ted was clearly still in love with Robin, and had only recently ended his first long stint in a relationship with her. In some ways, Stella was a rebound - and in typical Ted style, he was so desperate to be in love he managed to pretend that Stella was 'the one'. Clearly, this should have been a shorter, less intense fling for both of them, and all the signs were there.