There is a tipping point beyond which type 2 diabetes gets a positive feedback loop, which can make it extremely hard to treat with diet/exercise/drugs. Also, the beta cells in your body that produce insulin can wear out through exhaustion at having to produce so much insulin. Once that has happened, then injecting insulin is necessary.

Some people falsely think this means that your type 2 diabetes has progressed to type 1, but that's not true - it is still type 2, just more severe. Type 1 is a completely different disease, but with some similar symptoms.

I quite understand the "personal failure" part of this. You inject insulin if the diet/exercise/drugs weren't good enough to stop the disease progressing.

However it should also be noted that a decent number of people do actually develop type 1 diabetes late in life. Typically they will be misdiagnosed with type 2 diabetes, because doctors tend to assume that diabetes late in life must be type 2. If this happens, a progression on to insulin injections is nigh-inevitable, regardless of how much diet/exercise/drugs they have. There are a few tests that can help distinguish these patients - firstly they tend to be thin, but secondly the auto-antibodies that caused the destruction of the beta cells can often be detected in their blood.