The first two parts of your sentence are correct. The last one not necessarily correct. Depression can be caused by a chemical imbalance. It can also be caused/exacerbated by your own thoughts and approaches to life. It doesn't just happen it has causes and is correctable and in many circumstances is preventable.

I think the general point you were trying to make is good. Depression can crop up in the best of us. But acting like it's unavoidable and just something you have to live with is perhaps not the best way of approaching it either.

Oh, I guess he just wanted to point out that it 'happens' just as 'flu' happens or any other disease: it is not something that you try to get and in most cases it is hard to prevent.