People are not happy unless they feel that they are needed by others.
Statements
conditionals (only = right, unless = "if not")
people don't feel → people are not happy needed by others
quantifiers (most / almost everyone) causal (because)
knowing your job don't feel needed can be done by ⇢ other than family 1000s of people & friends
Evaluate
This one is kind of a mess to break down technically, because there's causality, conditional logic, and quantifiers. So let's get more conversational with our understanding.
Usually when we see a conditional rule in Inference, they want us to chain it to another conditional or apply it to some specific fact. Both of the first two claims are talking about Indispensability / Feeling Needed. Overlapping concepts point us towards possible inferences.
Since people don't feel happy if they don't feel needed, and since they only ever feel needed within their group of family / friends, it means that they don't feel happy outside their sphere of family and friends.
The 3rd claim provides a causal relationship between whether or not we feel needed and whether or not we think we're easily replaceable.
Goal
Let's find a safely worded answer making use of our knowledge:
Most people feel replaceable at work, so they don't feel needed, so they don't feel happy. (Within the sphere of family / friends, they may feel needed, so they may feel happy)