I've heard it said that to be truly happy in a relationship, you don't need money. Love will handle all of the heavy lifting and keep a relationship afloat. Cute, but ultimately not entirely true, in my opinion. I think there's a level of naivete that is associated with this line of thinking.
That said, even though we may still love the person we're with, if they're broke it will inevitably put a strain on a relationship for obvious reasons.
For instance, society, still, largely contributes to the notion that men should be the providers in a relationship; this in spite of the fact that women take to the work force rather than staying at home more now than ever. Many women still want a man that can provide for them, or at least take care of themselves financially and be somewhat fiscally responsible. So if a guy is broke all the time, that can make the woman feel insecure about the relationship's success and direction.
If one or both people in a relationship are broke, then that also causes an issue with other stuff. Most notably, being cheap. Now, I realize that some people are, by default, cheap asses even though they don't need to be. However, if you're being cheap then that has the affect of showing that a person is placing a lower value on their significant other, that they're really not trying. I feel that one can be frugal and still show ingenuity and value and making their special someone feel, well, special! Frugal versus being cheap is the difference between McDonald's value menu and picking up some snacks from the store and going for a picnic to a park or elsewhere, I feel.
Another natural side effect of having financial hardship is that it can stress a person out. This can be evident when out on a date. If a person is worried about something else, it will show; a person may not be into the date entirely, which will tank the chances of making a good connection. Nothing sucks more than being on a date and paying for a bill that you have no cash for and have to put it on a credit card. Unfortunately, these thoughts can be easily projected in a number of ways in a relationship like couples becoming distant from one another, easily irritatable.
If one should decide they don't have money to go out, then they'll make due and stay home more frequently which can possibly detonate a person's social life and even a relationship. For most people, there's a desire to be social, or even just to go out and experience life. Being broke can hamper such a life style, but it doesn't have to. Obviously there are any number of things a broke couple can do on the cheap. It takes some effort and research though.
Money isn't everything and it's certainly not the only thing, but it's a pretty damn important thing…