I have an answer at for How did Bruce Banner become the Hulk at will?, which answers how he is able to keep his intelligence when he is the Hulk. In short, from the previous Incredible Hulk movie (2008), he trained himself so he could willfully turn into the Hulk, and as it turns out, maintain control.
As for what Bruce says:
I don't believe he literally meant that he is angry all the time. It was said in jest or more likely a catchphrase before punching and bringing down an entire enemy ship.
As to how is unable to keep his intelligence, I would attribute it to
Loki's staff that was in Bruce Banner's vicinity on the Hellicarrier (ship). I believe Loki's intention of being captured by S.H.I.E.L.D. was to get the staff near Dr. Banner to induce everyone near it to become angry and hostile towards one another. Couple that with the ship being attacked, Bruce would then involuntarily turn into the Hulk, which he would have no control over.