There are some very interesting anthropological reasons why humans create these rituals and observances.
First of all, they create a spirit of "sameness" among people. We observe the birthdates of our friends and family members to remind each person that they have a special place in our lives. We celebrate anniversaries to look back and remember young love and see the wonderful results of a marriage of many years' duration.
Holidays such as Christmas, Thanksgiving, Easter, etc. provide an opportunity for renewing relationships with friends and family members we may not see regularly, for reminiscing together, for strengthening bonds of "sameness" among people who feel they belong together as a family.
When groups such as the WTS begin demonizing these observances and openly discouraging them, and even punishing people for participating in them, in effect they are erasing those potentially significant and meaningful bonding experiences. What happens next, very insidiously, is that the WT leadership creates surrogate rituals that revolve around the group, not around family. Among JWs, they include meetings, field service, circuit assemblies, district conventions, international assemblies, Memorial, baptism. The strength of the family bonds thus are a product of adherence to the artificial rituals created by the WTS. Once JWs begin missing meetings or skipping service, it's made very clear that your loyalty to the group is questionable, and by extension, loyalty to family becomes questionable.
For more information, I started a topic on this a few months ago: Traditions, Rituals and Celebrations are Important to Families