I want to become a werewolf again. Why? Well, think about it.
Being a vampire decreases your health, magicka, and doesn't let you regenerate stamina, and any fire, even a torch (no, just kidding, but you get it) can kill you as you get 100% more fire damage, unless you keep feeding on people, getting bounty (unless you feed on bandits or dark brotherhood targets, etc.) .

Now, being a werewolf INCRASES your stamina (you hear that, vamps?!), makes you move faster, and lets you turn into a blood thirsty carnage wild beast and even gives you a new perk tree.

So, yep, I want to be a werewolf. Now my history:

I started the game as anyone else, without knowing that there were werewolves or vampires on the game. Kept going on the game, until I saw the companions fighting that giant. I didn't even care. Later, some guards said that they knew werewolves exist, because they hear the howlings. No other town guards said that, so I started investigating Whiterun. I kept going on the companions quest line until I became a werewolf, and finished it. Later, I started the dawnguard DLC, and Harkon would kill me if I didn't accept to become a vampire. I wouldn't have time to turn into the werewolf, he's too strong, so I had to accept. I finished the dawnguard quest line, and then asked Aela to turn me back. Okay, I'm a werewolf. But then I'm fighting some vampires at a Dark Brotherhood mission, and a random one infects me. I didn't know that I would become a vampire if I didn't cure the disease. After I became, I though: "Okay, I just need to ask Aela to turn me back again." But the option is not there anymore.
That's it.

And in Solstheim, there is a group of "hunters", that after saying "leave us alone" a billion times, start attacking me and three of them turn into werewolves, so what do I have to do to make them talk to me?

Thanks for your input and humble opinion towards the Vampires vs Werewolf underworld debate wars.But I will humbly disagree with you as everything in Skyrim and most TES games depends on what level you are. Sounds like you've already been a werewolf twice and have been cured as Aela only gives the blessing but twice and only twice. Guess the best thing to do is to start over and rejoin the companions.

1. Being a vampire and being a vampire lord are two distinct conditions. Being a VL also gives you a perk tree.

2. Aela will RE-infect you with lycanthropy only ONCE. Or, to put it another way, will infect only twice.

3. If you were a werewolf, you should have been 100% immune to disease, including sanguinare vampiris. There are some exceptions however. Try having Falion cure your vampirism and see if you revert to your lycanthropic self.

4. The Frostmoon Crag hunters will welcome you only if you are a werewolf yourself. There is much to be gained there for dedicated werewolves, by the way.

Do you mean when Harkon offers to make you a vampire lord when you first meet him during your first trip to his castle? If so, you can turn him down and he will not hurt you. Because you just returned his daughter to him, he will not hurt you, simply banish you from his castle with the promise to kill you should you return. The only reason to become a vampire in the DG questline would be is if that is what you want - perhaps to go through on the vampire side instead of the vampire hunter side. Unless this somehow all changes if you are a werewolf when you first meet him. . . I don't know about that.