why am i asked for security questions when i never filled any out when registering my device
When i now try to purchase something from itunes store i am asked to answer security questions for which i never gave answers to when registering my device
You should have been asked to set them when you created your iTunes account (unless the account was created prior to them being introduced), not when registering a device to your account.
Are you able to set them, or reset them via the Password & Security section on your account on http://appleid.apple.com ? You won't be able to reset them if you don't have a rescue email address (which is not the same thing as an alternate email address) on your account.
If you can't set/reset them then you will need to contact Support in your country to get the questions reset.
Contacting Apple about account security : http://support.apple.com/kb/HT5699
When they've been reset (and if you don't already have a rescue email address) you can then use the steps on this page to add a rescue email address for potential future use : http://support.apple.com/kb/HT5620
Or, if it's available in your country, you could change to 2-step verification : http://support.apple.com/kb/HT5570