How to answer why suddenly we need indexes or query needs to be changed
How to answer what changed question by dev ?


This is a very common question not only with DEV, it applies to every team in IT and business.

What changed ? ==> can be answered by facts and figures.

Facts refer to for example 


increase the amount of users that are accessing the database ?
Any change in server configuration parameter ?
Database Maintenance - update stats, reorg/rebuild of indexes not being performed ? Due to this the plans are being incorrectly generated !
Amount of data has increased ?
Changes were made on the network side, OS was patched and/or a new service pack or CU for sql server was deployed - without doing a full regression testing of your application business cycle ?
The underlying SAN has become suddenly slow ? 


Figures can be derived if you have data to show. For example :


Baselining your server is critical during this situation. This will alleviate the blame game since you can support the facts with solid figures.
Start collecting data using DMVs or sp_whoisactive to a table, so that the data gets persisted after a sql server reboot.


(you have to workout based on your environment and needs, on how often to collect the data/ what data to collect and how much will be the retention period) or (you can invest in a third party software like sqlsentry or idera's diagnostic manager that will do the above work for you).