The speaker thinks it is necessary.

Someone else thinks it is necessary.

• I must buy flowers for my mother
  (It's her birthday and I decide to do that)
  
• "You must take more exercise"
  says the doctor.
  (The doctor thinks it is necessary).
  
• I must ask my secretary to book a flight for me. 
  (It is important for me not to forget.)
  
• "Dogs must be kept on a lead"
  (Written on a sign in the park = 
   a rule which must be respected)

• I have to buy flowers for my mother-in-law. 
  (It is not my decision - 
   my husband asked me to do it.)
  
• I have to take more exercise.
  (doctor's orders!)
  
• I have to call the travel agency. 
  (My boss asked me to book a flight.)
  
  
• I have to keep my dog on a lead.
  (That's what the sign tells me to do.)