Why Java needs Serializable interface?

Serialization is fraught with pitfalls. Automatic serialization support of this form makes the class internals part of the public API (which is why javadoc gives you the persisted forms of classes).

For long-term persistence, the class must be able to decode this form, which restricts the changes you can make to class design. This breaks encapsulation.

Serialization can also lead to security problems. By being able to serialize any object it has a reference to, a class can access data it would not normally be able to (by parsing the resultant byte data).

There are other issues, such as the serialized form of inner classes not being well defined.

Making all classes serializable would exacerbate these problems. Check out Effective Java Second Edition, in particular Item 74: Implement Serializable judiciously.

I think both Java and .Net people got it wrong this time around, would have been better to make everything serializable by default and only need to mark those classes that can't be safely serialized instead.

For example in Smalltalk (a language created in 70s) every object is serializable by default. I have no idea why this is not the case in Java, considering the fact that the vast majority of objects are safe to serialize and just a few of them aren't.

Marking an object as serializable (with an interface) doesn't magically make that object serializable, it was serializable all along, it's just that now you expressed something that the system could have found on his own, so I see no real good reason for serialization being the way it is now.

I think it was either a poor decision made by designers or serialization was an afterthought, or the platform was never ready to do serialization by default on all objects safely and consistently.