A common justification people use to rationalize their decision not to keep Shabbat is that what they’re doing is relaxing or fun, and therefore permissible in the spirit of Shabbat. But Shabbos observance depends on keeping in line with objective rules, not subjective standards.
There are a few problems with the common question, “Why can’t I use my iPhone on Shabbos since it’s relaxing?” [And its variants e.g. Why can't I drive/play music etc...]
1. First, the idea of the Jewish Sabbath isn’t relaxing or having fun. [That isn't to say you can't have fun on Shabbat, just that it's not the underlying concept.] The point of Sabbath is taking a break from the week’s activities.
The name itself tells us that: “Shabbat” comes from the verb “Lishbot” which is to stop work.
In modern Hebrew, workers going on strike are “shovtim.”
This is based on the verse we cite in the Kiddush for Shabbat “Vayishbot bayom hashevi’i mikol melachto” – And He ceased on the seventh day from all His work.” So the central concept of Shabbat is ceasing work and resting. As that same verse says of G’, “Vayanach” – And He rested.
2. The second problem is a psychological slippery slope. Disregarding the Shabbat to have fun involves the same actual activities as disregarding it for my job. If I figure it’s OK to call my friend on my iPhone to catch up, why not call a client to help build a strong relationship?
If I can drive to the cinema, why not drive to play a round of golf with my team at the office?
Observing Shabbos (Shabbat or whatever you want to call it) has a certain set of rules which make sense in light of ceasing work and resting. It’s not because something is fun or not that it becomes permissible – or impermissible. Fun and relaxation are fine on Shabbos – within the framework of keeping Shabbat’s rules.
For more on Shabbos observance, here are some valuable resources: