I see good information, but I'd like to add a chapter to this.
The key to their comments is the word "intense". Ask any married couple with 10 or more years, and though they will tell you their love is stronger today than it was the day before, they will also tell you that the intensity of those feelings has died down.
Before you go into red alert mode, intense feelings do not make the relationship. That strong connection of love is what does. Strong and intense do not mean the same thing, but they are confused for the other quite a bit.
When two people begin a relationship with someone and it begins to sink in that it's going to last, the two people's feelings are fresh, and exciting, and new, and it drives the two people to the max. As time moves on, the intensity will decline, but if the relationship is sturdy, those intense feelings are gradually replaced by that strong, solid connection of love.
This doesn't make the relationship bland, or common place. It's just that the two people have become secure in the relationship, it's a constant in their life, and it is not necessary to give it that nonstop attention.
Think of the friends you have. When you first begin associating, it's exciting and you talk all the time, and do a lot of things together at the start. Eventually you spend time with others, and then hang out with that person, and you guys don't have to talk all time because you know each other well and it's not necessary to discuss everything all at once.
This is the same idea with a relationship involving love, but there are major differences in communication and expectations.
It's my firm belief that the guys you've dated have hit that point of a strong relationship, and because they have little or no experience, don't know what to do when those feelings take over from the intense ones.
I would suggest that you not allow yourself to enter such committed relationships, at least in this age range, because you will have more of these if you do, which will only hurt you and make you question yourself more. Really, neither person is doing anything wrong, it's just a lack of understanding when it comes to a long lasting, committed relationship.