In my opinion, the value of some things is not high enough ... the possible reason is ... in my opinion, I don't pay enough attention to something. The possible reason may be ...

Secondly, the problem may not be (our system), but (our attitude). But maybe the problem lies in our attitude, not our system.

There are many factors that cause this result, but the following are the most typical ones. First of all, … there are many factors that can explain this result, but the following is the most typical. First of all, ...

There are many reasons for this situation. The following are some typical reasons. There are many reasons for this situation, and the following reasons are typical.

There are similar reasons when we consider ….