Laravel SQL Join请求将某些数据转换为NULL

I have 3 tables : users, travels and favorites.

I'm trying to write a request to retrieve data from db with the following informations : the travel, which user proposed it and how many times the travel has been marked as favorite.

This is my code :

  $travels = DB::table('travels')
            ->join('users', 'users.user_id', '=', 'travels.user_id')
            ->leftJoin('favorites', 'favorites.travel_id', '=', 'travels.travel_id')
            ->select('travels.*', 'users.user_firstname', 'users.user_date_of_birth', 'favorites.travel_id', DB::raw("count(favorites.travel_id) as count_favorite"))
            ->groupBy('favorites.travel_id', 'travels.travel_id')
            ->get();

It's almost working, except that travel_id value is now NULL when the travel is not marked as favorite (not present in the favorites table). I'm doing a leftJoin because I want ALL the travels (even those who are not marked as favorite).

What am I missing ?

Thanks a lot