Being jaded is one of the most horrible things to be. If you hear your players saying "Oh its just another dragon" or "Bandits? Its always bandits" they are getting bored with the usual fair. Even the most fantastic of settings can become common place and the most vile of places the usual.
One problem is when you get to higher levels. Unless they have a in world goal beyond getting stronger it gets boring. The higher you go the farther between levels and the more you need to grind to get there. Now you could start a new campaign with fresh characters but some people like a little more completion for their characters. The best way to achieve this is with them starting with some end goals. Do they want to retire after they get some land and a nice castle? That is a good goal and in older editions is already included in the game. Maybe they want to aim higher and become king. Whatever it is that they want to do let them aim for it and when the players agree that they have achieved it wrap it up and let the characters retire. If you play in a consistent world then it works even better because their new characters will hear about the old characters or may even have them as their benefactors.
Another problem could be they have just been playing to much of the same stuff. Some people may be fine with D&D all the time every time but not every one is. This can be fixed with some one off modules in a different setting. This could just mean taking out your copy of tomb of horrors and letting them go at it. Another way would be a different system altogether. Maybe if they are tired of fantasy pull out you Traveller book's and letting them go in the galaxy. Basically just getting them out of their rut with something different from the usual.
Finally it may be that the problem is something on your end. If you are always throwing bandits at them then their comment of "Bandits? Its always bandits" is justified. Maybe you have just been using the same random encounter tables so long that they could read it off line by line. This is easy enough to fix, just change your random encounter tables. The problem may be that at a certain level there are only a small number of viable choices in the books you have. With this just dig around online some. There are so many free resources out there that not matter the setting it is almost guaranteed that you will be able to find something.
To finish up I will say that sometimes if the group is bored with what you are doing the best answer is to throw the ball back into their court. Have someone else take a turn at DMing. If someone comments that they could make a more exciting adventure let them. Even if they don't take you up on your offer you lost nothing. If they did take you up on it though then they get to see it from the other side of the screen and nothing makes someone appreciate what you do as much as making them take a walk in your shoes. Plus you may end up creating a new DM and good DMs are always in short supply in my experience.