There are all kinds of things that can make a good relationship turn bad. Of course, the word 'bad' is subjective, but it's a rare couple that can maintain that initial excitement for the duration of their relationship. To put it another way: boredom kills relationships. As a guy, it's important to think of how to be a challenge to your girlfriend. But, just as 'bad' is a subjective word, so too is 'challenge'. Holding on to that thought; let's explore this idea a bit further.
Assuming that boredom is one of the main enemies of a good relationship, then it only makes sense that you will do whatever you can to keep things interesting. Anything goes, in theory; in reality, you need to be careful that you don't take your challenges too far.
What we are really talking about here is avoiding a rut, or getting out of it if you're already there. Some guys will try to ignore their girlfriend in the hopes that it will make her curious as to what's going on. The problem with this tactic is that it's dishonest, and it has a tendency to backfire in unexpected ways. One moment you're playing the male equivalent of "hard to get" and the next moment you're wondering why she left. You can certainly try this technique if you like, but if you do be sure to do it in small doses and pay close attention to her response to it.
A good exercise is to think back to the way you felt early on in your relationship. What was it about her that got you excited? What about you excited her? What would you talk about? What would the two of you do together? What things did you do to win her over? Answering these questions will not only give you clues as to what has gone wrong, but will also give you ideas on how to be a challenge to your girlfriend.
Try to do things differently. Yes, it really can be as simple as that. Change little parts of your daily routine, but be careful that you're not just creating a new rut. Keep changing things and keep your girlfriend guessing. She may not notice some of the things you're doing, but you can bet that the long-lost air of excitement will start to make its way back into your relationship.
The other thing you can do is talk to your girlfriend about your concerns. To be fair, it's not always easy for guys to talk about stuff like this, but you can do it if you really care about her. Let her know that you don't want her to be bored, and that you don't want her to think that you take her for granted. If you approach it the right way she will be glad that you brought it up, and she will be happy to offer suggestions of her own as to how to be a challenge to your girlfriend.