Difference between thread initialization and thread declaration in C++11

Originally I have this code:

 class Sender{
  public: 
    void deQRequest(){
        //some code 
   }
  private:
   std::thread t([this] { this->deQRequest(); }); 
}

I couldn't compile, got "expected a declaration error", I changed the code to the following:

class Sender{
public: 
    void deQRequest(){
        //some code 
    }
   void startThread(){  
        t = thread([this] { this->deQRequest(); }); 
   }
private:
  std::thread t; 

}

it worked.. what's the reason for this?