The way a date acts in little situations becomes an indicator of how they'll respond when something really important happens in the future. So if someone is constantly emotional or negative, even when a date does what they can to reassure... they aren't going to believe things will get better in the future. They will get sick of walking on eggshells.
Trying to fix things by talking about working on the relationship is a big mistake. A far more successful way to sort things out is to go ahead and do fun and enjoyable things together - not talk - to know it's working. Having big conversations about the relationship will simply make things worse if you aren't spending enjoyable times together in the first place.
Have you ever had someone say they care about you, or maybe even love you, but they admit they're not in love with you? The fact is that most people want someone who brings something better to their life, not take away their time, energy and emotional stability.
So when a partner doesn't have much going on in their life besides the relationship, and their happiness is defined solely by having that relationship, it's a big red flag for the future.
Have you ever said this to yourself after a break-up:
"I can't believe how I lost touch with my friends while I was with that person."
"I can't believe I let them control me like that."
"Where did my life go?"
"What happened to the real me? I wasted so much time in that relationship, when I could have been doing things for myself or my future."
The reality is that no person and no relationship can or should be everything to you. You shouldn't have to sacrifice all your time and energy on a partner. So concentrate on yourself first and foremost, and your future relationships will be so much easier, not matter how strange that idea seems right now.
