The context of the given sentence is to convey a particular quality of the listener 'you' that makes everybody love him/her.
Option B - 'Cheerful' means 'joyful, happy or light-hearted' which is a quality everybody likes. Therefore, this adjective is suitable to complete the given sentence.
On the other hand, Option A - 'Lugubrious' means 'gloomy, sad or depressed', Option C - 'Pensive' means 'thoughtful, dreamy or serious', Option D - 'Sad' means 'dull, unhappy or sorrowful'. None of these qualities are liked by everybody.
Therefore, the appropriate adjective to complete the given sentence is Option B - Cheerful.
The completed sentence, thus, becomes: Everybody loves you because you are so cheerful all the time.