In some environments, or for some ticket types, there always needs to be an Asset. After all it doesn't make much sense to have an install software request without a computer seleted. For those options you can create a validation rule that will require this option.
If creating your validation rule for something such as as "Request" Type just enter it with the following parameters:
Name: Asset must be selected
Comment: Requires workers to enter select an Asset when none has been selected.
Display this message: Please select a valid Asset. The Asset NONE is not valid for requests.
When: Every time incident is saved
When ALL of these are TRUE
"Type" is not equal to "Request"
"managed_object_id" is the same as "1"
To enter the last line of that parameter just use an <Advanced Condition>