The dwarves have to trust Gandalf’s decisions. Bilbo Baggins doesn't look much like a burglar, so I understand why the dwarves doubted Gandalf's words on him. Bilbo is a small Hobbit who prefers not to leave his house, and when you are looking for someone who will play a crucial role in a quest, that’s not usually what you look for. The dwarves have to trust Gandalf’s judgement about Bilbo so the quest will go smoothly. Of course, Gandalf was correct in his judgement, Bilbo was an essential part of the endeavor. This can be seen when Bilbo saves the dwarves from the spiders. Without Bilbo, the dwarves wouldn’t have made it out alive. Another time the dwarves had to trust Gandalf’s decisions is when he left them outside the forest. They have
