When Van Helsing explains what truly scares Count Dracula, she says its something that explains all his "smaller" fears or restrictions:

 - fear of the cross
 - fear of the sun
 - entering private property without an invitation

His biggest fear seems to be fear of death and thats why he keeps on sucking that girls blood, because _"she seems to be not afraid of death"_ 

The thing is, I can accept that the cross represents the commitment of Jesus to die, and the sun that we believe it will kill him scares him... 

But **how does this bigger fear explain why he can't enter without an invitation** ? 

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Note: as seen the Sun is not really harmful for him. Is part of the folklore that even he believed to the point of immense fear. 

Could be similar with the invitation. Either way, that he absorbs the myths is not just the **one explanation** that Van Helsing wanted to find as one unique truth