It could be that the filling is quite large and not strong enough to withstand the chewing pressure over the long term. I'm surprised it broke so fast though but is possible its very large. I would have assumed he would have warned you it could break soon though
It's really tough to answer this without checking the teeth present condition. Just want to know few question without answering your query. Did the tooth break & the filling stay as often, with a large filling this can happen? Does the dentist taken the x-ray to check whether it affected nerve of tooth? how much tooth structure is left & the height of the bone around the tooth remaining?
Since it's only been a week, I hope you're not going to be charged for having the filling redone. If the original filling was done correctly and the tooth is that weak, a crown would be a good way to strengthen and protect it.
Yes as what Jerome Tsang said if the filling is large and you are trying to chew hard objects it can get broken. You have not told us what your filling was a metal or composite material. May be the dentist just wants to make money from you and used a low quality material. If that is not the case you can go for the crown cover.