Expert: Prince William's love for Kate Middleton stronger than ever as he celebrates his 'bittersweet' birthday.

As the heir to the British throne, Prince William is celebrating his birthday with a "bittersweet" feeling, while remaining a "rock" for his family during a health crisis.

The Prince of Wales, who is 42 on Friday, has maintained his royal duties while his father, King Charles III, and wife, Kate Middleton, are fighting cancer.

On Saturday, the Princess of Wales bravely attended Trooping the Colour, marking her first public appearance since her diagnosis was made public earlier this year.

"According to Christopher Andersen, author of "The King," William has been an incredibly devoted husband to Kate. This is evident in the way he has taken on additional responsibilities to reassure the children and help with some of his father's duties while Kate undergoes chemotherapy."

"William acknowledges the impressive graciousness and insight in Kate's public statements about her cancer battle."

William has been more devoted to his ailing wife than ever as he looks after their three young children, according to Andersen.

Andersen explained that William understands that he is the only one responsible for reassuring the children that their mother will get through her rough patch, as well as their grandfather, who is also battling cancer.

"Andersen stated that George, Charlotte, and Louis are also very close to their grandfather the king, and people often forget about this. It is William's responsibility to ensure that his siblings are reassured about their mother's situation and their beloved grandfather's. The weight of this responsibility is significant as William turns 42."

In March, the Princess of Wales disclosed that she was undergoing chemotherapy for an undisclosed type of cancer.

The princess, 42, stated ahead of the military parade that she is making good progress, but as anyone going through chemotherapy knows, there are good days and bad days. She added that she has a few more months of treatment to face.

Kate stated that she is "not out of the woods yet," and officials emphasized that the engagement does not signify a complete return to public life.

In February, Charles, 75, revealed his cancer diagnosis and has now resumed his public responsibilities with less intensity.

Andersen said that this moment must be bittersweet for William, who, for the first time, has to consider mortality issues related to his wife and father, which he has never thought about before.

Despite ongoing health issues, the couple will celebrate privately with their children.

"William and Kate most enjoy spending milestones like birthdays and anniversaries with the children, just like the rest of us do with party favors, gifts, and balloons. Don't forget, Kate's mother founded a children's party supply company that was immensely successful."

William, a hands-on family man, will prefer to celebrate with his wife and children in private, according to royal expert Ian Pelham Turner.

"Perhaps the children will rally around and create breakfast in bed, roll up their sleeves in the kitchen and bake for their father," Pelham Turner told Planet Chronicle Digital. "Kate has a passion for baking birthday cakes and stays up to midnight using large amounts of cake mix and icing to produce over-the-top cakes."

Helena Chard, a British photographer and broadcaster, stated that Prince William cherishes family time and enjoys spending it with his wife, his rock, and their three children.

"William desires nothing more than peaceful, private family time. We hope Princess Catherine also has a good day and enjoys baking a special homemade, intricately decorated birthday cake with their children — a wonderful way to celebrate and conclude Prince William's hectic schedule."

Despite his royal duties and supporting his father, the Princess of Wales' well-being is still William's top priority, according to several royal experts.

Hilary Fordwich, a British royals expert, told Planet Chronicle Digital that while the prince has recently had to handle engagements on his own, he and Princess Catherine are now more in sync than ever.

"Since her cancer diagnosis, their marriage has become even stronger due to mutual support. They value family time and will celebrate his birthday together with their three children, prioritizing family above all else."

According to Pelham Turner, it was evident that William and Kate's love and affection were unwavering in front of the thousands of royal spectators during Saturday's Trooping the Colour.

"Richard Fitzwilliams, a royal expert, praised Prince William's unwavering support for Catherine during a challenging period. He is an exemplary role model as the future king and a devoted father. On this special day, we wish him a happy birthday and acknowledge his commitment to service."

The pair first met while studying at St Andrews in Scotland. In their 2010 engagement interview, Kate admitted that due to her shyness, it took some time for the pair to get to know each other.

In 2002, they moved in together with friends. A year later, in 2003, they moved in together and kept their relationship private. It wasn't until 2004 that their romance became public.

In 2007, they split and got back together. William proposed in late 2010, and they married at London's Westminster Abbey in 2011.

Kate mentioned in March that William had been a supportive spouse while she tried to cope with her cancer diagnosis.

"William's presence provides comfort and reassurance, as does the love, support, and kindness shown by many of you. This means a lot to us both."

The Associated Press contributed to this report.