One way a person may feel alone is if they do not have a solid support network like family or long-term friends who they can truly rely on. If a person has lost their significant other to death and has not been able to meet people he/she is able to trust, because they are dishonest or abusive, then this can lead to that sense of loneliness. I think a person can only go on trying for so many years before beginning to feel the "why bother?" creeping in. Then there may be more sitting around reminiscing/daydreaming and less believing they will ever find a good partner again.
Some people have families, too, that are disconnected with family members who don't leave the house or who say they will be there to help, but when it all boils down to it they are no where in sight. This can leave a person with a sense of knowing what it's like to be completely alone--meaning no companionship or assistance with day-to-day necessities--particularly if the person has experienced a serious, debilitating illness with a lengthy recovery time. Loss of physical stamina and capability places people in vulnerable situations where they begin to feel less capable (because they are) and it can take a long time, sometimes years, to build physical strength back again.
Take all of this and add to it the difficulties one faces when getting older in a society that values airbrushed bodies and that short window between 18 (hopeful not younger) and about 26 years of age when it is easier to feel carefree (for most). It seems at that age partnerships materialize somewhat easier and sometimes happen out of no where. Everything can be more carefree as opposed to the hopelessness one might feel when life has been far tougher on average.