So the main benefit of a meta description equivalent to ad copy on an ad. It gives the user some more information and makes them more likely to click.
Unlike titles where Google will typically respect what you give (your page is probably about one topic and your title will be better than what it could generate automatically), a meta description (as you pointed out) is far more likely to be changed by Google if it thinks it can find something that fits the users context.
For example suppose you have a post on the prices of cars and you have a post which talks about:
- Ford car prices & Landrover car prices
If that's the most relevant post on your site for car prices, Google will probably try and pull the Ford information for a user searching about Fords and a Landrover for the person searching about Landrovers.
You couldn't possibly write a meta description to fit that, but it can help users with those kinds of sub-topics. (In your example it looks like you've searched for "dreading christmas" (judging from the bold) and it's pulled the part of the post which mentions that.)
There will however, always be a set of queries which do get your custom meta description (in the above example it might be a generic one about car prices) and for these queries the question is: can you find a neat way to automatically or with minimal effort summarise a topic?
Doing it automatically will always be hard, I would at least set a meta description equal to the first couple lines of the main post (like Joe mentioned), it will hopefully prevent Google doing anything dumb like pulling in a menu and then pick your top posts and try and summarise those.
It's not the most high return activity, but worth doing if you have any particularly valuable posts.