Why Domain Names are Expired?

[arnoldap]

I have come across almost all facts and technicality of web hosting terminologies. But really not clear about a general question. Why the domain names are expired? Can we reactivate them again?

[weiphos2012]

That is a good idea. It does seem to help SEO of the site and should let you get more traffic.

[alanwalters]

Why the domain names are expired?

That's because the domain holder forgot to renew it timely or they don't want to use the domain name.

Can we reactivate them again?

Of course, you can .
The first 30 days of the expiration date is called automatic renewals of the original price.
The next 30 days called redemption period .
At these two periods you can reactivate them.