It wouldnt be for the purpose the make the relationship better. The point isnt to make it better but to accept it for what it is and be content with that.
Of course a deeper connection would be ideal but that may never be found. Have been looking for many, many years and everyone I connected with deeply did not feel the same way about me that I did about them - and that was only about 2 ppl anyway throughout all the years.
How do you know how deep a relationship should be in order for marriage?
It's a personal judgement call really. If you aren't happy or you feel like there's something missing, I personally wouldn't go through with a marriage in that situation. But that's just me. I'm a fairly independent person, I'd be fine living alone, I could support myself, and I don't want kids (unless they are of the four legged variety). I can afford all the time in the world to wait until I find the kind of man and relationship that I want. I'd be happier on my own than I would settling for something less than what I want. You are obviously a different person than me, so you have different needs and values that are driving you to potentially make the decision to marry this man even though you feel you could do better.
I would still recommend breaking this off and waiting, just for the sake of your own future happiness. But it is your call and you need to really sit down and think about what you want vs what he offers you. There are 7 billion people in the world, the chance of you finding somebody you connect with are higher than the chance of never finding somebody.
Also remember that if you marry him, you have to be able to truly be content. You have to choose to love this man every day, to be loyal to him, to want him. You have to live with this decision, and in the face of meeting a single man who you connect with better than your husband, you still have to choose your husband and not be resentful or bitter.
In regards to your last question, I guess the only thing I can really say is that you just "know". That's almost all anybody can say when they find somebody that they want to spend their life with. I've never been able to really gauge the deepness of the connection I've had with a guy. It's either been there, or it hasn't. The "semi-good connection" I had with one guy was not a romantic connection, it was a friendship connection and it took me 8 months to figure that out. I am very glad that I got out of that relationship. If I were to have married him, I would have been settling and I would have definitely been unhappy in the future looking back on it.
The guy I'm with now, the connection was there and we "knew" from the night we met. It's been my best relationship thus far and I can only shake my head at my past self for that last relationship and even considering marrying that person.