Our genomes need a regulation to control whether and when a gene should be expressed. In most of cases, it is done by changing the methylation and acetylation status of our histone.
We can briefly divide methylation into two groups, maintenance methylation and de novo methylation. The former one, which ensures daughter cells present the same methylation patterns as their parent cells do, is in the charge of the DNA methyltransferase DNMT1. After birth, methylation is introduced by DNMT3a and DNMT3b, which allows some particular genes silenced or expressed as the situation dictates.
Because of this change, may be you and I and every one else are same. In short, its beacuse of several factors that the epigenomic profile alters. One basic examples is the activity of the methylating enzymes. As pointed out by Zhang, it is the work of the DNMTs to maintain this profile: "Epigenetic memory" as many call. But, several studies have found that the efficacy of these enzymes varies based on their cellular titre or even SNPs that may alter the functional characteristics. More over, several toxic and lifestyle stress experienced post natal and till death keeps on altering and undulating the the epigenome. For further studies, you can refer some good reviews on this ever increasing and "dynamic" field.