Is there a significance as to why bitcoin mining "costs" time/energy/money? I have heard that the bitcoin mining process was created in a way that purposefully required the expenditure of resources (energy/money) in order to add real value to bitcoins. In this way, when more miners join, the value of bitcoins would rise because there would be additional resources that would be used to process the transactions. However, the way I see it, it is the opposite. In other words, it seems that the value of a bitcoin is based on supply and demand from buyers/sellers, and the degree of mining interest is based on this pre-established value. My reasoning is that people do not suddenly decide to buy or sell bitcoin because "oh there is more mining power going into validating the blockchain so it must be worth more." On the contrary, miners enter or leave the mining process based on whether the potential reward for successfully mining a block is "worth" the work, with the value of the reward being strictly based off the current market value of a bitcoin. Would anyone like to confirm or refute the latter hypothesis?
I have heard that [...] when more miners join, the value of bitcoins would rise because there would be additional resources that would be used to process the transactions.


I do not see why more validation power would make the currency unit more valuable. But before this discussion, lets clarify the technical side.

In general, the idea behind the Bitcoin protocol is to achieve a consensus view on the common transaction ledger in a decentralised way, i.e. no third-party intermediary to act as a clearing agency etc.
To avoid double-spending of coins, transactions need to be verified by miners before insertion into the blockchain - also, all nodes will synchronise with the longest chain available, so effectively the system works on a majority-based consensus system (majority of the miners determine the global chain).

Now, this would open up to security problems - one example is the Sybil attack, where an adversary simply creates countless virtual identities who all vote for his (potentially false) chain.

This is where the Proof-of-Work (PoW) scheme used in Bitcoin's mining protocol comes in. It is purposefully made computationally difficult for miners to verify blocks. Because of this, miners need to invest resources (energy, money) in order to participate - the rationale is that it is much harder to fake computational power in the network than simple identities, preventing a Sybil attack.

Back to the value of Bitcoin: Note that the following is rather subjective, and highly controversial.

I argue that it is simply determined by market supply and demand. Following this basic logic, the value of 1 BTC must increase in the number of users to join the network, since the monetary base of Bitcoin is limited by design. Because this is unrealistic in the long run, this argument can not be final.

On top of this, we can see extreme levels of speculation on cryptocurrency exchanges, driven by developments such as


adaptation (e.g. recognising Bitcoin as legal tender in some countries, more vendors' acceptance...)
news flow on upcoming updates, increasing awareness of cryptocurrencies and the resulting interest by the public and corporations
negative economic/political activity in some economies might lead to more demand for bitcoin



  On the contrary, miners enter or leave the mining process based on whether the potential reward for successfully mining a block is "worth" the work, with the value of the reward being strictly based off the current market value of a bitcoin.


Yes, this is correct. A causality between hashing power and the market value of 1 BTC is not apparent, other than that new miners will likely participate in the system themselves, increasing demand. Rather, as the market value increases, mining becomes more attractive in the short run. Note that after each 2,016 blocks, mining difficulty is adjusted to maintain a fixed mining rate, so then it becomes more expensive and miners will have to decide if they continue participating.