使用依賴屬性的原因：link http://stackoverflow.com/questions/1723756/why-dependency-properties
This helped me understand the reasoning:
The main difference is, that the value of a normal .NET property is read directly from a private member in your class, whereas the value of a DependencyProperty is resolved dynamically when calling theGetValue() method that is inherited from DependencyObject.
DPP是動態讀取進來的，而不是像原始Property那樣，實際是存儲在一個private的成員裏。
When you set a value of a dependency property it is not stored in a field of your object, but in adictionary of keys and values provided by the base class DependencyObject. The key of an entry is the name of the property and the value is the value you want to set.
DDP這貨是存在DependencyObject這個基類裏的。
The advantages of dependency properties are as follows:
好處如下
Reduced memory footprint（減少存儲，只存改變的）
It's a huge dissipation to store a field for each property when you think that over 90% of the properties of a UI control typically stay at its initial values. Dependency properties solve these problems by only store modified properties in the instance. The default values are stored once within the dependency property.
Value inheritance（值繼承）
When you access a dependency property the value is resolved by using a value resolution strategy. If no local value is set, the dependency property navigates up the logical tree until it finds a value. When you set the FontSize on the root element it applies to all textblocks below except you override the value.
Change notification（改變時候有提醒事件，Notification）
Dependency properties have a built-in change notification mechanism. By registering a callback in the property metadata you get notified, when the value of the property has been changed. This is also used by the databinding.
From: WPF Tutorials.