'Assign using active assignment rules' checkbox always checked by default

To resolve in Salesforce Classic:

1. Go to Setup 
2. Navigate to Customize | Cases | Page Layouts 
3. Click Edit next to the relevant layout. 
4. On the Layout console click on Layout Properties
5. Un-check on the 'Default' against the 'Case Assignment Checkbox' to disable
6. Click OK and save the changes to the Page Layout

To resolve in Lightning Experience:

1. Navigate to Setup by clicking on the Gear icon and clicking Setup
2. Click the Object Manager tab
3. Select Case and click Case Page Layouts on the left
4. Edit the relevant layout
5. On the Layout console click Layout Properties
6. Un-check on the 'Default' against the 'Case Assignment Checkbox' to disable
7. Click OK and save the changes to the Page Layout

Notes:

The same steps can be used for Leads object. In steps above, select 'Leads' in place of 'Cases' 

For example:
 

To force case assignment rules, enable the 'Default' checkbox and disable the "Show on edit page" checkbox under Case Assignment Checkbox. This will remove the ability for users to turn off case assignment when editing or creating a case. If both checkbox options are enabled, the Assignment checkbox is displayed when creating and editing a case and is checked by default.

NOTE: Unlike the Case object, the Lead object requires the 'Show on edit page' checkbox to be enabled when the "Default" checkbox is enabled.

NOTE :  When dynamic forms is enabled for leads,  'Assign using active assignment rules' needs to be enabled from Lightning Record Page.

The default setting will force the Assignment Rule to run unless you also display the checkbox and a user manually deselects the checkbox.

For more information you can also check the Page Layout and Assignment Rule documentations.