It's one of the regular mistakes of some people learning English, when they have an aversion of grammar and "theory" or they prefer to ignore simple explanations. But let's be clear, we're not passing judgment on anybody here. To me, it's just an indication on how we need our musts reinforced, be it sometimes by a preposition.
That should serve as introduction for a little piece of my own, we'll call "You must".
You must :) to (!) be secure, not arrogant, mature, not young or old, playful, not playing with me, complex, not complicated, spoil me without spoiling me and I will do the same, make me feel independent, not alone.