Everyone is different; Everyone has different hopes and dreams; Everyone reacts differently to every situation and feels different emotions.
Just as we are all different, so are the reasons people feel lonely.
From an early age we start learning/understanding that we need time on our own, just as much as we need intimate/quality time with those we love. We start to realise quite quickly how difficult it can be to balance time with our loved ones against time for ourselves.
At various stages in life, we reach milestones in life, that often accentuate these needs and make us vulnerable to our feelings and emotions making us feel insecure or lonely: adolescence; adulthood; thirty-something; mid-life crisis; menopause; and old age are often all linked with changes we feel to both our physical and emotional being.
Remember each of these phases will pass, and while it can feel painful and scary at the time, it can also be an opportunity to become more self-aware giving us a change to improve our overall wellbeing as a result.