Most advice about whether to do a PhD is written either by people who loved their PhD and can't imagine why anyone wouldn't, or by people who hated it and want to warn you off. The honest answer is more complicated than either perspective.
The Good Reasons to Do a PhD
You genuinely want to know something that isn't currently known. You've thought carefully about the research question you want to pursue and find it compelling in itself, not just as a means to a credential. You've experienced research through a master's thesis or research assistantship and enjoyed the process — the reading, the designing, the analysing, the writing — not just the outcome.
You want a career in academia or research, and a PhD is required to enter that career. You have secured funding that makes the financial costs manageable. Any one of these is a reasonable basis for proceeding.
The Questionable Reasons
You're not sure what else to do after your master's. You want to delay career decisions. Your family or supervisor expects it. The credential sounds impressive. You want to be called "Doctor." These aren't necessarily fatal reasons, but they're not strong enough to sustain you through four or more years of genuinely hard, often lonely, frequently unrewarded work.
The Real Costs Nobody Mentions
The financial cost is significant: even with a stipend, PhD students almost universally earn below market rate for their skills and age for several years. The social cost is real: friends advance in careers, buy homes, start families at a different pace. The emotional cost of constant evaluation — every piece of work scrutinised, every paper rejected, every defence a performance — is substantial and cumulative.
What the Career Prospects Actually Look Like
If your plan is an academic career, you should know the numbers: there are far more PhD graduates than academic positions. Most PhDs do not end up in permanent academic employment. This is not a reason not to do a PhD, but it's a reason to think carefully about which skills you're building and what the non-academic career path looks like from your specific doctorate.
Talk to People Two Years After Their PhD
Not to supervisors, who may be biased toward academic continuation. Talk to people who finished a PhD in your field 2 to 3 years ago. Ask them what they wished they'd known. The candour of that conversation is worth more than any generic advice.